In terms of Game Boy games that are fun, but expensive ($40+) to get a legit physical copy of:
-Mole Mania
-Mega Man III (best for expert Mega Man Classic series fans)
-Mega Man IV
-Mega Man V
-Pokemon Red, Blue, Yellow, Silver, Gold, Crystal, Trading Card Game
-Mario's Picross
-Avenging Spirit
-Shantae (although it's now available on Switch)
-Wario Land II
Since the Game Boy is region free though, getting Japanese copies of Mole Mania, Mega Man III-V, Mario's Picross, and Wario Land II would be a cheaper alternative too with the language barrier not being a big problem.