>>22
I think you shouldn't go.

First, it will probably be hard for you.
Seconds, she will be disappointed that you can't go. "disappointed" + "you not there" = win.
Thirds... don't be her puppet. If there is the slightest suspicion that she invites you because you're a "last resort of some kind" and she's sure you'll accept... don't do it.

Well anyway. Tell her you can't go (invent a reason - ie you're already going out with friends, or whatever - if it gets evident that you stood home just pretend the whole thing has been cancelled, or I don't know, find something). Don't be harsh, don't be rude, go something like "awww, I'm so sorry, that won't be possible for me... maybe another time, but I hope you'll still have fun!". She mustn't feel that you're evading the thing at all.

Note that this method also gives you a reason to invite her later to said amusement park, so it won't seem awkward if you do; and as you are asking and giving place and time, you are in control. Girls, most of the time, like that.