I'm a big fan of the Hyqueous Vaults (3rd level adventure written for OSRIC), and it seems like it would be a good fit for your hexcrawl. The PDF is free on Lulu.
The Pathfinder adventure path Ruins of Azlant is set on an uncharted island, with opportunities to travel to nearby islands as well. I'm currently playing in it, so I haven't read it, but so far (parts 1 and 2) it has been great fun. It has lots of small dungeons dotted about the place and seems like it could easily be repurposed as a sandbox by throwing out the metaplot. (Admittedly the price per dungeon might be a bit high afterwards, so maybe not good value for money.)
The Paizo adventure Crucible of Chaos (for 3.5 edition) involves an interesting exploration of a crashed flying city. I'd be very tempted to include a method for the PCs to get it back flying again (for a short time, anyway), allowing them to access places they otherwise couldn't reach.
The 2nd edition adventure Evil Tide has an interesting dungeon in it and the plot is easily removed.
Barrowmaze is a megadungeon, so not what you are looking for, but if you happened to own it already then the above ground environment and "lesser" burial barrows can fit nicely in a sandbox without needing to include the main dungeon.