Open RCT2, open RCT2, open RCT2.
Right now I'm just designing pre-built rollercoasters in order to fill out my quick options when I'm playing the "real" game. I'm focusing on small, cheap coaster types since you really only have the ability to build expensive coasters after you're well on your way to beating the scenario.
While there's already plenty of pre-built wooden rollercoasters, here's a pair in designed that nest inside each other.