What's a nice way to let someone go
That you love but you know you can't be with?

Like if it's there last few days and you kind of had a relationship thing going with them. But nothing happened and you want to leave it all on good terms?
Do you tell the person how you feel and so all the best
Or do you just act like whatever and say bye bye?

Just sit them down and tell them, straight-out. It's really the best way; that way there's no confusion, no leading them on, no issues with them trying to decipher non-existent clues they think you're dropping.

It'll hurt no matter how you do it, but this way it will hurt less and if they're mature and well-adjusted, they'll handle it.