I too have used time travel plots many times. But the PCs in my games tend to use their time-travel powers to just gain "extra time" in their "present," for example a recent item creation gig which was to take 5 days of down time ended up taking about 6 seconds in game time as far as the main plot was concerned, because the PC with the time travel power used it to jump the party back in time 25 days, to when they knew the area they were in was both sealed off and empty, and then Teleport to a remote wilderness area, use a Magnificent Mansion to avoid drawing any attention even
there, and proceeded to go through the down time outside of ordinary reality. And at the end, the procedure was reversed- Teleport back to the sealed chamber, then time-jump futureward to six seconds after the party left the first time.
But for truly weird fun, I've made liberal use of the Region of Dreams, which I borrowed ideas from Wheel of Time and other sources to make more than what it is in the basic
Manual of the Planes version. I normally run "Dream Sequences" which represent "special dreams" the actual PCs themselves have while sleeping, in-game, and we allude to the Dreams during regular game sessions. The Dreams play out like any other game session really, with the characters playing themselves, though they're almost never in a place resembling their "current" (meaning, when they go to sleep) surroundings and the Dream landscape sometimes never resembles anything the actual PC ever encounters in the "real world."
Where the weird fun comes in here, is that I make time a fluid and unpredictable thing in these Dreams, so that PCs often get hints of planned future campaign events in them, meet beings they wouldn't otherwise meet in them, and otherwise get to do really crazy stuff. Another thing I've done on several occasions is allow players who have had multiple PCs, or are playing in different campaigns of mine set in the same world, to use their alternate PCs for the Dream instead of the one which should be in the party with the ones having the Dream- so I get campaign crossovers and chances to say goodbye to old friends that way. On one occasion, the PCs having the Dream actually carried the crossover into the "real world," by having one Dreaming PC from one campaign send a letter to where the Dreaming PC in the other campaign said he was, and since both characters remembered the Dream that would have weirded out most players quite enough. I went further though- the two campaigns were on slightly different timelines at the time; one of them was taking place a couple of weeks before the other, and so what I had happen in the "real world" was that the letter arrived for the recipient character the day after he had the Dream- but the kicker was that in order for the letter to arrive that way,
it had to have been sent at least a week earlier in game time.

So the PCs had their Dream interaction, and both had the same Dream, but they had it two weeks apart.
Boy that was fun.
Another thing I did was have an NPC I expected to have the party meet in the future, show up briefly in the game at one point and greet them like they were old friends, before vanishing through a portal to the Far Realm. When the PCs actually met this character some months (in game time and real time both) later, the NPC didn't know them and was meeting
them for the first time, from its perspective.

Ah, the joys of tangled timelines.