Whenever I travel I allways bring my running-shoes with me. Ofcourse there a days I saw a lot and don't want to run but even a short (about 30min) run can make me fresh.
Also a nice thing is to go to a sauna of even a beauty-salon for a refreshment.
Swimming you can allmost everyware, some swimming pools have also a sauna.
Big tip - don't feel you have to try every different type of food you come across. I did a contiki where at least all the girlson the coach put on about a stone and I lost about 3 kgs because I was more interested in buying souvenirs and walking around the sites (it helped that i had been over this way for a year before i did it and the novelty value of the market stalls was considerably lessened though).
Most of all try not to deviate from your regular patterns. You will find you eat later in the day but you can control that most of the time.
And make the most of your feet by walking everywhere. Most Euro-destinations will have allt he good stuff fairly centrally located so there is no need to take buses everywhere!