Well you can always change the time if you want - as long as you don't care about time-specific things for the time being (like berries and whatnot) which would be affected by the time-changing, you can use that to your advantage to refight people.
There's also your rival at the E4 at certain times/days, Red reappears after each time you beat the E4, and there's other trainers to call too which after a few battls end up being pretty decent so you can call at least someone at any specific day and time afaik. There are no other options afaik though bar wild Pokemon, and that's boring.