I don't want to do something with friends that we've been planning forever
What is the event?
I don't want to go on a group trip because I think it will be a waste of money
It would be rude to back out at the last minute, especially if you've already committed to the plans. Even if you don't think it would be fun now, you might really enjoy yourself with your friends once you go on the trip.
I'll be fine. I know all the popular tourist spots, I can entertain myself.
I think that you should stick it out. You made this plans so long ago and it would be rude to back out now.