An I spy is always a safe choice.
Ones my DGS or other boys we know have liked - construction vehicles, Cars, superheroes, hot air balloons, puzzle pieces, dinosaurs, Pokemon, Sponge Bob, cats & dogs, airplanes, sailboats, teddy bears, the playmat panels with roads to play with matchbox cars, zoo animals, Clifford the Big Red Dog (supposed to be a book), cowboys and horses. . .