I recently discovered a kind of round castanet that you can play by placing them on a table and beating them them with your fingers. (Can't do that with the traditional Spanish design). I don't know what their intended use is - schools maybe? If you have some experience in playing drums with your fingertips or fingering woodwind instruments, this gives you a head start - the rhythms are just there, no need to think about what gymnastic motion might be needed to land on the beat. I used these a few times at Whitby, some people on this thread might have seen me. The downside is that you need a flat surface to sit them on. One stall had Whitby had a box of plastic ones for sale (varying a lot in sound and quality).

A few weeks ago we had a band from Majorca come into Sandy Bells. One of their instruments was a huge and incredibly loud set of castanets - they had built-in resonating chambers. Each one was the size of your fist. I'd never seen those before.