Themepark is generally referring to a worldbuild method where you have a bunch of separate microcosms that don't interact. Think disneyworld: you've got a bunch of distinct areas, but some mascots will simply not ever wander into a different area.

Here are orcs, here are dragons, this is a fire dungeon, welcome to elf forest, etc... and the players are the only people to really go from area to area and once they've experienced it, they'll likely leave it for another and unless there's a big shakeup, you won't see the orcs mingling in dragonland.

Sandbox is generally referring to a more open experience where the players are free to go about and build and destroy things as their capabilities allows: some players bring a bucket, some bring a shovel, others a watering can, etc... and they're left to play. But as a whole the sandbox campaign is based around what the players want to do and the GM helping them reach those goals, akin to leaving a kid to play in a sandbox, the parent being there to supervise and make sure they don't hurt each other.

You can have a Themepark Sandbox... they're not mutually exclusive.