As far as meeting people/things to do a few thoughts -
Because of ISU, you'll always run into loads of college students. However, there are a fair number of 20 - 30 something working professionals (& grad students) in B-N as well, they're always there, you just notice them more during ISU's breaks

. Just a few thoughts off the top of my head -
1) If you're into more physical/outdoor activities, there's a bike/walking trail (Constitution Trail) that runs through the center of B/N. There's also a rock climbing gym that's located in an old grain elevator. Additionally, there's two nice lakes, Evergreen Lake & Bloomington Lake with some hiking, boating, biking, fishing opportunities. Both lakes are just north of B-N on the east & west side of I-39 (respectively). I think you can even rent boats/canoes at Evergreen Lake.
2) If you're into the bar/band scene, there's a row of bars (some with live music on the weekends) starting at the corner of market & n Main street in Bloomington. There's another row just south of the county courthouse along W Washington Street (between Madison & East st). There's, of course, alot of college kids that hang out here, but also a good number of 20 - 30 somethings.
3) For some art/indie/classic movies, theres the Normal theater (a old rennovated movie house in downtown Normal)
Normal Theater. There also used be to a theater like this in Bloomington call the Castle Theater, but I'm not sure if it's still open.
B-N, for some reason, seems to be the restaurant capital of Central Illinois, so there's PLENTY of places to eat.