Tree Fort

The Tree Fort is Finn, Jake, Beemo, Neptr, and possibly the Snail's home, and where they can usually be found when they aren't adventuring. It includes a living room, a bedroom, a kitchen, a pond, a well, and a wide look-out view.

The Tree
The tree itself appears to be completely hollow. It may not even be alive. The entire interior of the tree trunk is covered in boards. The canopy is not actually leaves. The entire thing is a wooden structure, coated with some form of greenery. The whole building may not actually be a tree anymore. It has been broken and repaired so many times, there may not be any actual tree left in it.

According to Natasha, the green is actually tree leaves. So the tree may not be dead.

Entrance / Treasure Room
The entry to the tree house is filled with all the treasure Finn and Jake have collected on their many adventures. Minus all the heaps of gold, the room is a simple empty room with a wood front door on one end, and a ladder leading up into the hollow trunk of the tree on the other.

The treasure in the entrance seems to include old TVs, VHS tapes, a unicycle, lots of gold, a Nintendo entertainment system complete with several games, and a few other unusual things.

The entrance can best be seen in the episode "Video Makers" when Finn is looking for a video camera.

Tree Trunk
The hollow trunk is completely filled with treasure. There is an intricate web of ladders to get around it. One of the ladders leads up into the canopy where the living room is located.

The inside of the trunk is very rarely shown because its so empty. But it was shown in “Guardians of Sunshine” when the whole front of the tree was blown out.

Living Room
The living room is furnished with a brown colored couch, several long tables, and a large monster-skin rug. In the corner there is an old phonograph and what looks like a wood burning stove with a pile of kindling next to it. This is the room where many of the typical activities that Finn and Jake occur, such as watching movies or playing video games on Beemo. There is a clock on the wall that says, "Finn and Jake Timeless." The Living Room is next to the Kitchen.

There appear to be two living rooms-- one with a long brown couch, round red couch, and cooler; and the other with a purple chair and monster skin rug. The rug room is located one floor above the couch room.

Finn's Bedroom
Finn's room is over the living room and is accessed by a ladder made of planks nailed into the Tree Fort's tree's trunk. On Finn's bed is his Eskimo-style sleeping bag on top of many pelts of various evil-looking furred and scaled monsters, assumed defeated by Finn himself. Finn dislikes worms crawling on his bed, as shown when he threw a book (possibly The Enchiridion) at the worm. Next to the bed is an end table with what seem to be a mug and a rusted muffler on it, and beneath the end table is what looks like a large magnifying glass. Over the head of the bed, the jaws of a very large shark are hung on the wall and a spiked mace hangs from the ceiling. The skin of some unknown six-armed beast lies on the floor as a rug. A large tree limb growing through the room has an old battered desk on it and in the intro has Beemo set on it. They apparently keep a cricket beside Finn's bed in a jar as seen in "Her Parents."

Jake's Bed
Jake sleeps in an open drawer of a cabinet, which also holds his viola, a picture of Lady Rainicorn, books, bones, and some of his other personal possessions such as a mug which looks to hold milk, ties, and a locket of some kind.

Cave
A small cave is built into the tree fort on its back side. It appears to be where Finn and Jake keep tools and equipment for maintenance; it is here that Finn gains the material and equipment that he uses for "creating" Neptr in "What is Life?"

Attic
The attic is seen in "Rainy Day Daydream." There are the initials "AT" (that presumably stand for Adventure Time) carved into the tree, and a rug with various items such as candy and books. There is also a picture of Lady Rainicorn near the rug. It seems to be very dirty, presuming that nobody cleans it. Seemingly, the only way to get to the attic from the weapon room (the branch directly below) is by climbing into a bucket and using a pulley system to haul the person or objects up. Otherwise, using the bridge which connects to the living room will grant access to the attic. It seems to be an abandoned area since there are only a few items that are useful. However, it appears that Finn and Jake do use it sometimes. It also appears in "Go With Me" when Finn was talking about Princess Bubblegum hating him.

Yard
The yard has several features including plants, a pond, a well, a yurt (the round hut structure), a mechanical device that might be an electrical generator, and a vegetable garden in the back. The well does not have water in it. Instead it is the enterance to the Beneathaverse where the Gnomes rule.

Weapon Room
The Weapon Room, as its name suggests, is a room that stores weapons. It has a large gauntlet, a small gauntlet, and a small bomb that resembles a nuclear weapon. So far, it has only appeared in "Rainy Day Daydream." It also has a bookcase, a chair with a pillow, and a desk with a notebook near the large gauntlet. Other than that, there is not much in the form of weaponry. There are way more swords and weapons scattered around the tree house than there are in the weapon room.

Boat
Seen mostly in the episode "Slow Love," Finn and Jake keep a boat as a type of look out view at the very top of a branch at the top of their Tree Fort. It has a umbrella and other small objects in it. It also appears in "Power Animal" at the Roof Party.

Roof
The top of the tree that the tree house is built on.

Although the roof appears to be the top of the trees canopy, it is a solid surface. So it has been used for parties and star watching as seen in “Power Animal”.

Trivia

 * According to creator Pendleton Ward, Finn and Jake built the tree fort themselves, although the tree was originally owned by Marceline, as seen in the episode "Evicted!."
 * According to Finn, Science Cat and Shark appear here at 4:00 P.M. everyday.
 * On a clock hanging in their tree fort, there's a picture of Finn and Jake on the hour and minute hands (respectively), surrounded by the words "Finn and Jake--Timeless."
 * On the wall above the couch, (best seen in "The Jiggler" and in "Rainy Day Daydream") there is a ripped picture of a seemingly naked woman. However, according to Natasha, "It's not a naked woman, it's a woman in a crappy snapped bikini"
 * The Tree Fort is the only tree in the Grass Lands.
 * Finn and Jake keep piles of gold coins, gems, and other treasures in their house that appear near the door in "When Wedding Bells Thaw" and other episodes. Ironically, in "Wizard," when Finn thought he had to pay for the last magic lesson, he apparently said "You can't do this to me man, I got no money!" Although he could've meant that he had no money with him instead.
 * A few characters have paid a visit to the Tree Fort: Ice King, Marceline, Donny, Lady Rainicorn's parents (Bob and Ethel Rainicorn), and Lady Rainicorn.
 * In some episodes like "Slow Love" and "Guardians of Sunshine" the Tree Fort is nearly destroyed but then rebuilt.
 * The treehouse as seen in "Business Time" is completely different from the regular build in later episodes. This is likely due to the episode's original sneak-peak airing, during which the treehouse design was apparently unfinished.
 * The tree is likely not alive since it is hollow has openings and has been destroyed in two episodes.
 * It is shown in "Conquest of Cuteness" that there is a network of small tunnels with openings which the Huggaboopies used to their advantage to quickly move around and escape from the fort.