There is no better way to learn about robots and electronics than building your own. This Nerf Dart Launcher by Markus Puntz is a fun project that can give you a few ideas. It has 3D printed PLA parts, 2 motors, servos, and a few other simple components.
This Nerf Dart Launcher can move around fast to launch darts at targets. It can be used on a FPV rover or trike. You can find the instructions here.