The late game's grueling difficulty doesn't really mesh with the fun unpredictable nature of the units' actions and physics, and makes repeating many stages infuriating. Disregarding that, the Sandbox mode shines in the shear unfathomable amount of infinite combinations of battles you can create with the over 100 different units, and the pretty robust unit creator. You can possess every single one of them and they all have their own dumb-fun moveset(with quirks and all).

I'd say look up a guide for how to acquire all the hidden units, and just go bananas in the sandbox. The campaign is completely optional, while still showing occasional greatness.