I am going  to give you strange answer for this one. For sure the best summer vacations will include beaches or other water sources like lakes and stuff. I would like to suggest a different kind of tourism. I will call it "Off -season Tourism". Try visiting  in the summer  places which are great to visit in the winter (it doesn't have to be necessarily in bad weather conditions). In this kind of tourism you get to know the "real" life of the visiting location. Different cultures function differently when it is off season then when it is full of tourists. Another reason to visit at the off season is the existence of less tourists and cheaper prices. I must add that not all sites are good for this kind of tourism. Think about it...