Can I replace my waterbed mattress with a regular mattress?
My waterbed mattress sits on a pedestal with about a 2 inch rim around the bottom of it. I want a regular mattress again. Can I just put a regular mattress on that pedestal, or would I need to buy a box spring too?

You won’t need a box spring at all, in fact, it’d make your bed look abnormally high out of the frame if you did. I did it when I was a teen, got sick of a cold waterbed all the time so I got a regular matress instead.
You should be able to drop a mattress into the pedestal, but measure first. Waterbed mattresses are sized differently so you may end up with a big gap around the edges. A boxspring isnt necessary, though mattresses are designed to perform better on one. But I think it would make your bed too high.
I worked for a national mattress chain recently and had several individuals who purchased a waterbed in the 80′s come in to buy a mattress.
You don’t need a box spring. A Simmons mattress will work fine as long as it’s on a solid platform.
Measure the size of your the slot you intend to set the mattress inside. A regular QUEEN size mattress will not typically fit correctly. Consider purchasing a CALIFORNIA KING size mattress. This is a different size than the regular king sized mattresses and the makers of water beds in the 80′s conformed to this size.
You also may want to read this article about how to save 40% when buying a mattress.
http://www.infobarrel.com/Mattress_Wars_Buying_a_Customer
Hope the info is helpful.
John