I went to the UK. I actually went with my folks as a graduation present but even if you go with friends and do the hostel thing it would still be a great time. Wales has great beaches. London is well London and has something for everyone (including lots of cool medical museums if you are into that sort of thing on your off time). Dover is a cool little resortish cruise port/major shipping point that would be fun to stay to dink around in. Bath is expensive but also super fun. Lots of shopping ect. if you are female.
Germany is great for traveling too. A lot of the castles have great land and hiking trails (nothing like Yosemite or anything) but lots of fun. King Ludwig's castles are my favorite. Language can be more of a barrier in Germany than say Wales though it can be a problem in Wales too if you go too far North. However, if you stick to major German towns almost everyone speaks at least some English. Also you can easily drive to Switzerland or down into Italy depending on where you are staying. Compared to America everything is nice and close 🙂
I personally like cruises. Once you are on the ship the food is free and so there are lots less unexpected expenses. Just be sure to add in money for tips and drinks (tea and coffee are usually included but depending on the line that may be it). For summer I wouldn"t go to the Carribean... It is hurricane season. Your best bet is either Europe (more expensive) or Alaska (potential to be lots of retirees). However usually the staff is young and lots of fun. Everything is there and there are off boat excursions that give you lots of different options in terms of activities. For example in Alaska there is everything from white water rafting (pretty mild but still fun) whale watching, dog sledding or loads of historical tours. Kind of depends on what you are into.
Also something I have always wanted to do is go to South Africa. If you look online you can find relatively cheap package deals that look like loads of fun. Or more local suggestions... Go to a lake Tahoe and rent a houseboat. Go camping in Yosemite and hike (Yosemite is one of my favorite national parks but really any park is fun). Go to a dude ranch for a week (something my best friend and I have planned for when she graduates law school and I graduate med school respectively).
Good luck on your biochem test.
