It's no joke that sandbox games have seen an influx since the PS2 era, with Grand Theft Auto 3. However, GTA 3 wasn't the first game. Here's some fun ones that existed before then:

-Grand Theft Auto 1, London, and 2, of course. While 3D added a lot to the series, the games were still fun in 2D.
-Tony Hawk's Pro Skater 1, 2, and 3, in Free Skate mode.
-Pilotwings 64, in Birdman mode. Unlimited flight in any of the levels until you crash.
-Body Harvest for N64, made by DMA who later became Rockstar Games. It was Grand Theft Auto gameplay in a rural environment.