The way I see it, there are two types of time travel.
First of all, there is the time travel where you just speed up or slow down time itself. This is not really traveling through time, rather you are altering the speed of time. You would activate your time-altering device, and you and everything else would speed up or slow down. I suspect this may pose a challenge, seeing as you would have to alter time for the entire universe or universes. This poses an interesting question, though: could you speed up time for just one person or area? Time would go faster for that person, but for everyone else, time would stay the same? Any interaction between the two would certainly be interesting. Also, if your time-altering device should malfunction or break down, the alteration of time would stop, and it would go back to normal. There is no way to go backwards in time with this type of time travel.
The second type is the actual time travel in which you and your time machine stay the same and you move through time. Actually, everything else stays the same, but would have appeared to have changed because time had advanced(either forward or backward). It will still be difficult for this form of time travel, but for different reasons. You wouldn't necessarily have to change anything else, you would be traveling through time so everything else would change itself.
