What lie should I come up with for this issue?

I have a problem. My mom offered to take me and my friend Taylor to the beach two days after school lets out for summer. Taylor's grandpa owns the condo we were going to stay in and my mom already made reservations. He was going to let us stay there for free because we're taking Taylor with us (remember, that's his granddaughter) but now I'm not so sure I want Taylor to go. My mom said that if she can't go I can take my friend Ben instead, however Taylor nor her grandpa can find out (Long story). More than likely Taylor will be able to go though. That's why I need to find a way to talk her out of it. I'm not sure how because she's looking forward to it and has been for a few months. I can only go to the beach once this year since money is tight. How can I get Taylor to not go to the beach with me? I really want Ben to go.