Play Aperture Desk Job. It’s a free game on Steam that does a great job of introducing you to the various hardware doodads the Deck has crammed inside for controlling games. It’s also just fun so that helps.
How about the op Valve classics? Half-Life 2+Episodes and the Left 4 Dead duology all run great on the SD. Another good one is “Classic” Star-Wars Battlefront 2
Left 4 Dead 2 is a certified classic.