Since looking for a “just right” board a while ago, I’ve maintained a spreadsheet of every Arduino-compatible boards I can find, currently 147: Save your own copy for easy sorting and exploring! My current backlog of boards to put up is Menta, Femtoduino, and ReaDIYmate. If you know of ones not listed (or listed ones that are not available), please let me know!)
Make a robot friend with Adafruit’s CRICKIT – A Creative Robotics & Interactive Construction Kit. It’s an add-on to our popular Circuit Playground Express, FEATHER and other platforms to make and program robots with CircuitPython, MakeCode, and Arduino. Start controlling motors, servos, solenoids. You also get signal pins, capacitive touch sensors, a NeoPixel driver and amplified speaker output. It complements & extends your boards so you can still use all the goodies on the microcontroller, now you have a robotics playground as well.