Ok, here's some ideas on Time Travel. First of all: it will probably require a lot of bookkeeping, to keep it all straight, regardless of what way you will actually go with.
"Been there, done that."A lot of times, when characters travel in time, they will get to a scene that they have already been at, only from a different perspective. The declaration method covers this beautifully, and a player could always say "hey, we might get here later and help ourselves in this fight, without us now knowing about it.". However, you can easily compel them in exactly that situation to act the way they declared before.
I would recommend using a timeline to fill in as you go, to keep track of where the characters are and where they have been.
In general, this goes great with a random-time-jump story device that the characters get somewhere, and it should probably be connected to the big bad. Let them fight the big bad early, and let them defeat him while he is trying his time travel ritual (thus making the whole situation worse). The characters get sucked into the vortex and are thrown through time randomly, until they find a way to stop it. I would set a few (half a dozen maybe) fixed times that cover the better part of 2, maybe 3 weeks, between which they will jump randomly (at least to them, you might have a plan behind this). One of them could be in the future, where, at first, the failed ritual will have had a disastrous effect. You can roll the destination for each jump, for both the players and for the antagonist, so the way they meet will be random as well. The declarations will have great effect on how the other jumps play out, especially, because they see their way through the jumps linear but not the way the antagonist jumps. The declarations of one scene can influence the result of the next. I get dizzy even thinking about it now, but I hope you get what I mean
"One Year of Hell"The basic idea is, that the villain succeeds or something absolutely horrifying is happening, that the characters don't understand and therefore can not stop. At the end however, they will find a way to travel back in time to a critical moment and choose the other leg in the trousers of time. There are some tv shows that have done this, if you want to go all out, this is a great option.
"Groundhog Day"Plain and simple, ye olde time loop. can get tedious, if the players have to do the same things over and over again, just to get to a point of the day they can get into action, but of course you could skip those things after they are figured out to get to the more interesting parts. Day Break is a great inspiration for this type of time travel, I think.
And then there's the doctor, like tetrasodium said. It is full of a lot of time travel stuff, and the Bad Wolf example is a great one, especially if used multiple times, like they did in the series. I was really creeped out, when they brought it back.
Another interesting example would be the 4th season weeping angel episode.
And another one I like very much would be the season 5 finally:
The doctor and his companions are running from a dalek, when suddenly another doctor appears in front of them, shot, and collapses. He whispers something to himself and he seems to die, however, he only pretended, so he could use the time he bought himself (about 20 minutes, I believe) to save the universe.
That's what I got for now, I may come up with some more once I got some sleep.
