Paia Theremax
I've always wanted to play with a theremin and have had only a few opportunities to play a commercial Theremin instrument. Once was at a fantastic music museum in Phonenix where they had one set up in a room where you could interact with everything. I believe it was a Moog brand theremin if I remember correctly. Nice instrument, but WAY more than I am willing to pay for one.
I just happened that I came across an inexpensive used kit theremin produced my Paia, one of my favorite companies as it has always been budget friendly. It needed a small amount of repair and some tuning / calibration, but I had it working well in a short time. It came with no antennas so the first thing I had to do was make those. I used hanger wire which worked just fine. I also had to build some sort of case to hold the control panel and board, as well as the two antennas I had just made. I was so anxious to try it out I just found some boards that were about the right size and made an open top 'container' just to get it to work quickly. Below are a few photos of its current configuration, which is playable when connected to an instrument amplifier.
I was first introduced to the Paia theremin in the pages of one of my electronics magazines. It was a two part article that went through the assembly of the Paia theremin. This is why I love Paia. They were always putting their cool stuff out there in magazines and books essencially free if you were electronically inclined and had the desire to DIY a project of their design.
Here's a photo of the PC board and control panel as it is, attached to the board I'm currently using as a base or case. The power adapter is just one I had in my large stock of wall-warts that was the proper voltage and current capacity (and low noise) to work with the Paia. It's sitting there because it's hard wired in.
Here it's shown with the antennas deployed. The volume antenna on the left and pitch on the right, just the same as pretty much every theremin I've seen. The antennas just fold into the middle for easy storage.

I built a stand for it to sit on out of scrap lumber that positions it at standing height for easy playability.
This is the control panel. As you can see it has a lot of adjustability and jacks for quite a few external controls as well as the audio out for connection to an amplifier or mixer.
If you're not quite familiar with what the theremin sounds like, here is a short video of the most amazing virtuoso on the theremin instrument designed by Leon Theremin (Lev Termen). I don't know if there has ever been anyone who plays a theremin better than Clara Rockmore!
The Theremin is among the first electronic
musical instruments to be invented.