The Williams campus has many lovely trees which are great for climbing, if you like that sort of thing. Here are some favorites of the Williams tree-climbing community. Trees also make for great hammock spots.
 Magnolia beside Thompson Chapel
This tree is located between Thompson Memorial Chapel and Griffin Hall, in a sort of nook created by the chapel's buttresses. It's fairly low but provides for some nice, undistracted sitting, especially in early May when it is in blossom.
 Sugar maple in front of the physics building
This is one of my favorite trees to climb, located in front of the physics building and across the street from Jesup Hall. Getting into the tree initially is a bit of a trick; it requires standing on the railing dividing the sidewalk from the grass, grabbing the nearest branch, and swinging one's legs up onto an adjacent branch. (Alternatively, a good vertical leap from the base of the trunk can get you a handhold and you can swing your legs up onto the adjacent branch.) Once in the branches, however, it is quite easy to maneuver and continue climbing up. There is a nook about 20 feet up where three branches come together which is perfect for sitting in, whether to bask in the sun, take a nap, or do a problem set. It's also fun to talk to people coming out of the physics building and watch them turn about in bewilderment trying to find the source of the voice. However, in the spring, one does have to watch out for eastern tent caterpillars that tend to swarm on the branches.
 Red maple in front of the chem/computer science building
This is a lovely, large tree that is perfect for group tree-climbing parties, as it can comfortably accommodate quite a few people in its branches at once.
 Sugar maple in front of West
This tree is located just in front and to the right of West College as one stands at the door of West looking towards the science quad. It's rather small as trees go, and quite easy to climb. However, when its leaves are out, they are quite thick; anyone sitting in this tree will probably not be seen even by people ten feet away. A good place to hide, if that's what you want to do.
This tree, east of the temporary mailroom, is perfect for mulitple arborealists; its many accessible branches are kind even to inexperienced climbers.
 American beech in Forest Garden (between Stetson and CES)
You'll need a boost up to the first branch, but after that it's lots of comfy crotches and sturdy limbs. Depending on the season, there may even be fresh berries or other goodies to snack on growing in the gardens below.
 Oak near "Climb high" gate
 Maple next to Cost bench
 Copper beech in front of WCMA
This is a large, beautiful tree (the only tree of its kind on campus, to my knowledge) which would be impossible to climb were it not for some unfortunate disease or fungus which has rendered its trunk incredibly gnarled and knotted. As a result, getting up into this tree requires not so much actual tree-climbing skills as it does rock climbing skills. It's mostly fun just for the challenge of getting up onto the first branch, but once up it does afford a nice place to sit. It's possible to climb up higher into the tree, but not advisable without actual belaying equipment (which has, in fact, been done).
 Trees in the Stone Hill pasture
There is a stand of birch and ash trees (?) in the pasture on top of Stone Hill that makes for great climbing. The branches are low enough to the ground that you don't need any special techniques to scramble up. The branches on these trees are thick enough to make great seats/lounge spots, plus you get an amazing view of the campus below you and Pine Cobble rising behind it. If you're lucky, you might see the cows mulling around on Stone Hill, too!
 Maple just off path between Sawyer and Hopkins
This is a small tree and some may have trouble getting into it, but once in you can go quite high. You can't really sit in it, but it's fun to surprise people passing below you.
 Huge maple in the Science Quad
Start on the branch that hangs low towards clark/bronfman. you can jump up and grab it, and then ascend it upside down to the main fork which you use to right yourself and continue on to the main trunk.
 Maple on road down to Mission
 Tree between Woodbridge and Droppers
I don't know what kind of tree it is, but it's pretty easy to climb and quite sturdy, despite its flimsy appearance.