My younger son used to be heavily into trains and we slowly built up a small collection of O-gauge and N-gauge trains and built a couple of tables with dioramas.
By the way, we’re also amateur rockhounds. Geocaching and rockhounding are closely related and I would encourage you to give rockhounding a try. Here’s some quick links to get you started, the easy way, by visiting a local show: Go to the California Federation of Mineralogical Societies (http://www.cfmsinc.org/) and click on their Show link. It’ll list a calendar of upcoming rock and gem shows. One that I’ve made an annual trip to is the Victorville Tailgate Rock show – all prospectors at lower than retail pricing! Visit their link here: http://www.vvgmc.org/tailgate. If you’re in the Anaheim area, you can also visit the Anaheim Searchers Gen and Mineral Society (http://www.searchersrocks.org/).