Oh gosh, I had so many. I remember having a huge model train (HO scale) set in the basement. Dad and I would spend hours putting together little models we would get from the hobby shop down the road to add to the set. It was so fun building our own little city and watching the trains go around.
Another favorite were RC cars. I remember my first being the Bigfoot monster truck. Dad also had his own RC car as well - a red Jeep Wrangler (hmm, I have an actual one now) - but I would always try to smash into his. Needless to say my monster truck didn't last very long LOL.