The seesaw was constructed out of common lumber (2x6s and 2x4s), deck screws, a short iron pipe as a fulcrum, and some plywood, foam, and vinyl seats.  Size the length of the seesaw and height of the fulcrum to whatever you are comfortable with.  Mine were 10ft and 3 ft respectively.  At that size, I felt the handlebars were required for safety and provided a convenient place for the buttons.  The seats were 2ft long benches to allow for either multiple riders or sliding forward or back to balance the weight.  The electronics were equally simple.  I used a BlueFruit EZ-KEY HID module to mimic a Bluetooth keyboard.  The module gives you 12 pin outs that, when connected to ground, mimic a key press.  The two handlebar buttons were connected in parallel to the space bar pin.  I used two tilt ball switches arranged in an upward V pattern to detect when the seesaw was tilted to one side or the other.  These were connected to the up and down keys.   Experiment with the angle between the sensors so that you can detect the two tilt positions and the flat seesaw position.

[ I see now that the EZ-KEY isn't currently available.  So I would suggest using a Adafruit 32u4 and this example sketch for a BLE HID keyboard - https://learn.adafruit.com/introducing-the-adafruit-bluefruit-spi-breakout/hidkeyboard ]

Seesaws are fun by themselves but using it as a controller took it to another level.  The build is easily doable with a minimum of tools.  Please comment if you find another game that this controller works well for.