Most people are somewhat preoccupied during the summer anyway because they're here for classes or an internship. So what to do outside of work/class time? I will concede, you have to know some people who are also staying in town, or have folks your age at wherever you're working. The past week, I've filled my days with lunches and dinners with friends, some tennis in the afternoon at Snyder, movie nights (I'm currently on a sci fi and romantic comedy kick), yoga and dance classes at Studio 206 on the downtown mall, and just plain a lot of hanging out with friends.
Apart from what I like to do, there's lots of hiking and biking trails for the outdoorsy minded, Ashlawn Highland and Monticello for the history lovers, one hour drive to Shortpump + Stony Point and two hours from Tysons for those who like to shop, beautiful countryside for those who like the country, Corner and downtown bars and clubs for party goers, and so on and so forth.
Sounds great, but for those who are coming from a huge city (like I did), it's definitely a difficult transition. I got used to C'ville and love it now, but people who like the hustle and bustle and everything that can be found in a city would definitely find C'ville to be boring at first. It's especially bad in the summer time because if you don't know anyone who's left here, you'll be bored out of your mind.
Having just almost completely negated what I said in the first part of the post, I actually still love spending my summers in C'ville. It's a great time to rest and recharge from the school year simply because most of the students aren't here. It's a completely different feeling, and I enjoy having the place to myself. That's not to say the town just dies, there are quite a few people who live here irregardless of the school.
Bottom line is? You'll have to form your own opinion. But as a city girl (massive city, seven million people, yeah) who came here, I absolutely love it here. I'd want to live here when I want to settle down in a life. Great environment, friendly people, and still relatively cheap cost of living.