This was cute. {= )

My one critique would be that you can break up some of the longer paragraphs when the focus changes. For example, this one:

“Here we go again,” she grumbled, trying not to smile. “If I throw up on this one, I will blame you, James.” Click, click, click. She was strapped in. An attendant went around checking to make sure everything was secure, and it started. A chain pulled the coaster up a steep hill. Rebecca wasn’t too bad with heights, but this was somewhat higher than she liked. The coaster stopped.

I'd split that up like so:

“Here we go again,” she grumbled, trying not to smile. “If I throw up on this one, I will blame you, James.”

Click, click, click. She was strapped in.

An attendant went around checking to make sure everything was secure, and it started. A chain pulled the coaster up a steep hill. Rebecca wasn’t too bad with heights, but this was somewhat higher than she liked.

The coaster stopped.

As you can see, I second Linstar's suggestion of italics for the sound effect. {= ) But anyway, I'd use this paragraphing because, as I see it, there are four different points of focus here:

1) Rebecca's dialogue;
2) The dissociated sensory detail of her strapping herself in (a rare good use of passive voice—but do avoid it in general);
3) Narrative about the start of the ride; and
4) The moment of suspense at the top of the hill.

Other than that, again, cute! I wonder what Rebecca did to earn this vacation?

~Neshomeh

I didn't notice any technical errors besides the ones the other two have already pointed out.

Also… a vacation? For a PPC agent? :O

Poor Rebecca! Leave from work is supposed to be less stressful than work! I hate roller coasters, but I’m glad Rebecca got at least some enjoyment out of this one, somehow. Good of her to put herself through it in the name of watching her cousin.

Um, in the sentence:
Standing in line waiting, and then getting in and waiting some more while everyone else boarded and the coaster finally got itself going was torture.
I think there should be a comma after “going,” since the phrase starting with “and then” is sort of separated from the main action of the sentence.

—doctorlit does tolerate Big Thunder Mountain, though, because animals

I don't remember if you've written with James before, but I really liked this. I might have italicized the "Click"s myself.

--Ls