There are lots of handheld retro game systems, but the GameShell offers far more versatility than others. In addition to playing emulators for Atari, GB, GBA, NES, MAME, MD, PS1, and more, its modular construction and GNU/LINUX-based dev kit encourages you hack and modify to your liking. Some assembly required.
Folks who love the games on the Nintendo Switch, but who never play on their TV set will want to check out the new Switch Lite, which plays all the games that support handheld mode, but is strictly a portable. Its controls are built in, and the motion and camera abilities are gone. Drops 9/20/19.