Would appreciate any recommendations.
It includes both "hardware design" and programming. Hardware design in the form of balancing power usage vs capability. A big honking amount of solar panels is convenient, but then the mass of your grobot increases and you need a bigger more power hungry engine too. Software design in the form of a small but very capable stack based language.
One of the fun parts about grobots is you get more than one robot and cooperation between your robots is pretty important! There's a fair few published robots, ranging from simple algea-tactics (Just sit there and photosynthize) to bee-like swarms with workers and queens to stationary robots with big cannons that depend on a small and nimble scout to pick out targets at a distance.
Its an old site (java and .net), but it appears to still be going.
https://www.ibm.com/developerworks/library/j-robocode/index....
[1]: https://www.curseforge.com/minecraft/mc-mods/opencomputers