I don't want to go camping for my friend's 21st birthday.
Why don't you want to go camping?
I don't like the weather and I hate mosquitoes.
I totally get it, I hate camping too.  There's nothing wrong with not liking it.  Just make sure you let your friend know your reasons.
Nah, I just need to come up with an excuse to avoid going.
I think it would be best just to be honest with your friend and tell them you really don't like camping. I'm sure they will understand and would appreciate that instead of being lied to. Just offer to spend some time with them to celebrate their birthday when they get back.