For a suggested donation of only $1 you can check out a very cool Darwin exhibit at Philosophical Hall drawn largely from the collections of the American Philosophical Society's library.
The exhibit provides some background info on how Darwin came to piece together an understanding of evolution (a word which doesn't actually appear in Origin of Species), and includes a fantastic display of letters in his hand and some fantastic 100-150 year old naturalist books. There's also an art display in the steampunk style with manufactured parallel 'history' of Darwin's age, which is one of the few types of art displays in conjunction with something historic that I can really enjoy.
This is so refreshing following the non-scientific movie tie-ins the larger science museums in this city have been pushing for the cash, and in the wake of America's regressive creationist push toward repealing the 19th century.
It's the 150th anniversary of a brilliant man's brilliant work which pulled humanity out of a certain ignorance; time to celebrate!