I think I have a rough understanding of physics, at least up (or down!) to the quantum level. The details of string theory is beyond me, but this is probably true of most physicists too. I understand the concept of black holes but once again I don’t know how to calculate the Schwartzschild radius or anything else about them. I mention black holes because they are related to wormholes, which seems to be physicists’ preferred way to explaining the possibility of time travel.
Black holes also pose the problem of preservation of information: there is this idea in physics that information cannot be destroyed. I have the same intuitive (and perhaps false) idea that the energy/matter content of the universe should remain constant, which leads to my suspicion that time travel is impossible (since it would introduce two discontinuities in the energy/matter content of the universe).
However false my ideas might be, and however possible time travels turns out to be, I like to think about what would happen if I were – somehow – transported back in time. Let’s say I end up in 1650. Suppose that I could (1) not go crazy when I discover my new situation, (2) manage to stay alive and support myself, (3) learn to communicate with other people, and (4) retain all the knowledge I have now.
If I were convinced that my situation is permanent, I wonder what I would do? Firstly, I would probably say “screw history!” and not be afraid so introduce “modern” scientific ideas. But what would these ideas be? Would I be able to produce electricity? Perform scientific experiments? I don’t think I remember any experiments well enough to really make much progress. Apart from the fact that I don’t remember the details of Thomson’s or Millikan’s experiments, there is the question of obtaining all the raw materials to conduct the experiments. Astronomy is just “crazy talk” without strong telescopes. Medical knowledge might be useful (Harvey had discovered the circulation of blood by 1650, but Galen’s ideas were still influential) but would probably be roundly rejected. Chemistry would perhaps be a good area to work on, but once again raw materials and equipment pose a problem.
There are only three things I think I could realistically spend my time on: writing down as much general scientific knowledge as I can remember, writing mathematics, and composing music. I think my knowledge of mathematics is enough to establish at least the basics at a time when calculus has not been fully developed. Whether anyone would pay attention is an open question, but I think I would be able to establish my own name in this new history! 
As far as the music is concerned, I wonder whether anyone in 1650 would find modern music appealing. Considering the fact that much of Beethoven’s music was disliked at first, would anyone find my renditions of the Beatles, Gershwin’s songs, or Abba’s hits appealing? Perhaps I would concentrate on late classical music.
It is a silly thought experiment, but I still enjoy making it. If at all possible, I would definitely try to travel, but I doubt if I would appreciate tourism much: somehow our ideas about the value of places are a function of the time we live in.