How can we say no without giving explanations?

Let's say that a friend asks you if you want to go on a trip with him. You have a bad feeling about it, your intuition raises a red flag. How do you say no without making up a fake excuse? When I say "no, thanks for your offer but I don't feel like it." People tend to try to convince me, and I end up having to be cold. I can't realistically say "I appreciate your offer but my intuition tells me that this is a bad idea and I can't explain to you why."