Edit: BOTH MYSTERIES SOLVED :D Original post follows:
I've been trying to find a couple of books from my childhood, but I can't remember their titles or who wrote them. Maybe some of the 80s-born here can help me.
The first book (SOLVED - THE BEAST, BY R.L. STINE) I thought would be easy to find (or at least remember the title) is about two kids who ride a roller coaster after the carnival closes, and end up going back in time to the 1930s. They befriend another kid, impress him by buying 4 cents of ice cream with a nickel, and that's as far as I remember.
The next book I remember even less. (SOLVED - THE DEAD GIRLFRIEND, ALSO BY R.L. STINE) This one in loves a girl- maybe new in school- maybe not, who is met by a creepy kid who ends up being creepier still. This girl has all kinds of things go wrong, including being electrocuted by her keyboard in her computer class. She starts blaming (I'm not sure) either her rival or the creepy kid. At the end of the book, she throws a pArty and creepy kid shows up. She grits her teeth, doesn't want to make a scene, and offers him some pasta. When she goes back into the kitchen later, she finds her cat in the pasta pot.
That's all I remember, sorry.