Need an excuse! How can I get out of this?

Bad knees.

I always use that one. :)

Don't encourage her to lie and cheat,that is a very bad way to behave. She has to learn life involves doing things we don't like and that lesson should have been taught from the start.She wants the treat because it is a treat,there is no educational value in an amusement park so she must put in the effort. Tell her she has to do the hike,who knows she might enjoy it and she will certainly learn something about map reading, about the country side,the flora and fauna ,about pushing yourself ,about team work all very valuable lessons and of far more worth to her in the long term than a day at an amusement park.

Dentist appointment if she doesnt need a doctors note. Also you could write her a note saying you both will be out of town on the day they go hiking.

Bad parenting skills I must say. She hates hiking-so what. You are teaching her that it's ok to not do something just because she doesn't want to. That's going to end up biting you on the behind in the long run. You are also teaching her that it's ok to lie. Double bad on your part. Shame on you. She's probably a bit overweight too I'd reckon.

Dentist appointment

You do know that by coddling her and helping her LIE to get out of things she doesn't enjoy doing spoils her and starts inserting into her head a sense of entitlement and the attitude that "if I don't want to do something, I don't have to". By helping her get out of doing this, it's telling her it's okay to not participate in things because you don't feel like it or that you can simply pick and choose what you like. Good luck trying to tell her to eat everything on her plate. Way to set an example as an adult.

there will be a family affair like a wedding or reunion party

She's afraid of getting Lyme disease from ticks.

allergic to exercise ?