I have the whole "maybe I can only do exams" sometimes. But it's just you being nervous and scared. This year at uni (2nd year) I'm going to force myself to do so much more. The opportunities are there, at university, to do so much more.
It's not so much about how interesting and what you've done before uni, more so at uni. Just like Colleges only really care about GCSE results, and Unis generally only care about A-Level results, graduate employers only care about your life at uni. So who cares if you didn't go to summer school for something, I wont hold it against you if you have a good grade at uni and have done more things there.
Trust me, it's so much easier at uni. In one year at uni I've been simulation skydiving (in a wind tunnel), snowboarding in france and scuba diving in Malta!
Granted these aren't academic things, but I'm going to work on this a lot more this year. So my advice would be not to worry.
Try other summer schools. Why not write to companies/businesses for relevant work experience. What cours are you applying for, by the way? May help people offer better suggestions.
"Being interesting" on a personal statement can be as simple as reading around your subject, perhaps looking up some acaedmics in the field and seeing if they have any seminars you can go to (some free, some you pay), does your college have a debating club (if its relevant), can you get yourself in a position of responsibility (or do something like help out at a charity), work experience (as mentioned before) or take up a sport (can be team, or solo such as running).