Quick correction-- it's the red and brown lines that go the Clark/Belmont, not Blue.
That's one of may favorite neighborhoods, I highly recommend it. Gret comic shop there, too, if you are into that.
The taste can be pricely, but people-watching is free, and there are free concerts all weekend.
I believe the Top of the 'Cock (Hancock Tower) is free, and still one of the tallest buildings in the country, worth a look.
There's a ton a funny, cheap theater around, too. Check out Too Much Light Makes the baby Go Blind if yo ulike improv/oddball theater. it's not improv, but has the same sense of humor. Go early, it sells out.
There's also the beach, with more people watching (but less clothes).
EDIT: Dennis' Place for Games is also at the Clark/Belmont stop. RED LINE! Not Blue line.