It's always a little tough to say no, but sometimes we just have to be honest with people and let them know that we're not really feeling it. You could try something like, "I'm sorry, I don't think I should go on this trip." or "I don't think I'm up for it right now." No matter what you say, just make sure you sound genuine and honest. After all, they'll probably be able to tell if you're just trying to avoid hurting their feelings.