Looks like you are already biking? That's what lifted mine!
Yea it could really be good to bike but i think it needs the right seat. And i don't know the right one for this :D
If you want a tighter butt make a workout session full of bodyweight squats, lunges, half bridges, Bulgarian split squats. All of that with your bodyweight :)
But i thought squats will make your butt bigger?
It depends on the weight you're lifting and the rep range