The Sonic Potions LXR, as previously mentioned, looks like an interesting drum synthesizer - there’s a surprising amount you can do with it synthesis engine, as this video proves:
I was also interested in the Audiothingies P6 for a bit - a six-voice, DSP VA.
However: those are basically moderately complicated DSP-based kits. If you’re interested in much simpler, rawer, and totally analogue things, I cannot recommend enough getting stuck into the world of DIY guitar effects pedals. There is so much out there to build and tinker with, and many of them are dirt, dirt cheap. Also, because of the analogue signal path and low component count, they lend themselves to experimentation and learning what’s going on much more simply than a DSP-based box.
To that end, Tagboard Effects has long been a useful source - it’s now getting a bit ‘completist’ in some of the pedals they’re trying to layout, but the archive has all manner of fuzzes, distortion boxes, overdrives, even simple IC-based delays and choruses. Lots of fun to be had here.
And, to wrap it all up, I can’t recommend Nicolas’ Collins Handmade Electronic Music enough: a book on making electronic music from first principles, starting with mics and contact mics and then onwards to simple capacitor-oscillators and so forth. It’s wonderful.
Build contact mics. They’re brilliant fun.