Seismology is a great pastime. Geography, geology, physics, electronics, work in the shop, mathematics, and computer programming all rolled into one. And it runs in the background even when you have to do other things!
I built a simple Lehman seismograph in the winter of 2004. A short description of the seismograph is linked to below. I have added a small GPS receiver and software called NMEATime to provide a more accurate time base. The seismograph is oriented for E-W sensitivity and I am working on a second one to use for the N-S component.