We jumped the car and got it over to Chris's parents' house. We had to leave it running while we ran all our errands on the way so we wouldn't get stranded :) We wanted to test certain things and rule them out to find the problem, so we asked a couple of opinions, and then started with the battery. The results?
We ended up buying a new one! Chris and his dad put it in in the parking lot outside the store and Ben started up with new vigor! Turns out he'll live for quite a bit longer (we hope). Thanks for all your help, you gave us great places to start and questions to ask!
(If you can't tell, I was thrilled when my car started on its own!)