Hammocks are wonderful, portable, outdoor couches. They can be bought for as little as $15 at outdoors stores. A good hammock spot is often sunny, with smaller, sturdy trees about 8-10 feet apart, set back a bit from a path so people don't bother you too much.
Frosh Quad
Set up between the trees and the fence at the western edge of the quad. Probably the most consistently sunny spot, especially in the afternoon.
Mission Park
There are a number of trees near the Haystack Monument (especially on the Dodd side) that are suitable. Sometimes a bit shady for early- or late-season hammocking.
Sawyer Lawn
A few trees can be made to work over by the Congregational Church. The trees are a little too thick, and it loses sun in the afternoon.
Dodd Quad
There is a tree on the Lehman side of teh Dodd Quad that will take a hammock set up within its branches, instead of between two trees.