Well, one of the most enjoyable things about the drumming/percussion world, is the limitless experimentation that can be done with techniques, sounds and applications. There are truly NO limits. I couldn't possibly share everything I've done in this thread there are so many, but I'll share some things I've done with djembe specifically and the 'whys'.
Similar to you, I've used materials on the head to create a muted bass tone. It all depends on what sound I'm going for. If I want an electronic, Roland 808 bass drum sound, I might use a piece of rubber (for example, Bartman gave me the idea of using one of those sound-off rubber practice pads you lay over a drum to mute it) layed on the head. If I'm going more for a thunder type sound, I might lay a thin towel over the head and play rolls with mallets. If I want a snare type sound, I might lay a single caxixi on the head and play an open tone with the other hand. Sometimes, I'll strike the head directly with the caxixi itself. If I'm miced and am playing in a quiet environment, a cool sound effect is to take a tuning fork (usually to the key of the song, but not necessarily), strike it really quick on my knee and then lay it on the head letting it vibrate. Depending on the amount of pressure you apply, you can get some cool effect noises/sounds. Try laying some goat toes or shells on the head and playing around that or dropping/dangling them on/off the head.
Of course, try a variety of sticks/beaters combining them with your hands (i.e. brush in one hand with the other bare hand) for neat applications.
I hope this gives you some ideas. Make SURE to have fun. If you try something and don't enjoy/have fun doing it, move on to something else. With limitless possibilities, life is too short!
Oh yes, try doing a search or just doing some heavy-duty browsing here at the Café. There have been other threads like this one with MANY great ideas and approaches.
