LEGO Classic Large Creative Brick Box

The LEGO Classic Large Creative Brick Box (Set 10698) holds 790 pieces in 33 colors, including eight different types of window and door, six tires with wheel rims, and two green baseplates, the larger just over 6 inches square. Everything stores in the plastic tub it ships in, and the bricks fit every other LEGO set.

The classic no-brainer. Theme sets get built once and then sit on a shelf. This is 790 loose bricks with ideas included if a kid wants them and total freedom if they don't, which is why they come back to it for years.