The University of Dallas has an excellent summer program (Latin in Rome). You earn 3 college credits in 3 weeks while touring ancient sites in Rome, Pompeii, and Naples. Both my older daughters did the program and loved it! They found the quality of both the instruction and the student body (about 30 kids) to be very strong.
It's expensive (I think about $4-5K), but the cost does cover travel, food, lodging, instruction, etc. Scholarship aid is available, and in our experience has been generous (no full-tuition grants are made).
The only problem is that the deadline to apply is in February, I think. It takes a long time to get student visas and make other arrangements. If you aren't a junior and will still have time to do a summer program next year, you should check it out. Here's the website:
http://www.udallas.edu/travel/latin.cfm
If you're looking for something closer to home, how about checking the websites of schools that are known for strong classics depts. (Holy Cross and Bryn Mawr come to mind) and seeing what they have available for summer?