Stupid question: Are hex crawls a form of sandbox or something completely unrelated.
Not stupid...I don't really know either. Here's my rough idea...
To my thinking, if you have a small section of a world (with open borders) roughly key-out---with enough places to go that you can plop the players into it and let them decide when and where to go---then it's a sandbox.
In a pre-written sandbox, I'd image all the locales would have to be fully keyed. That limits it's size (I think), e.g.
Swordfish Islands, Through Ultan's Door, etc. The unsolved equation is then how to best present the interconnections and make it dynamic.
Most folks, I imagine, have a version of Bryce's DungeonLand---a rough framework of the "outside" that connects a bunch of pre-written site products (e.g. Frog God Games'
Lost Lands). Sometimes random tables are heavily leveraged to fill in details. The interconnections are minimal or solely the responsibility of the DM.
In a home-brewed one, (e.g. mine) it is fairly vast...but only roughly key in many places. Details get filled in after the PC's choose to explore in that particular direction (or I have a particular itch to scratch). The sandbox evolves and matures over time.
OK. Now what is a hex crawl? Just a hex-grid placed on the world map?
Alternatively, it's a framework for pacing the frequency of the randomly generated events as the party moves from hex-to-hex. In my opinion, the latter is not a sandbox---it's more like a computer game.
The beauty of a sandbox, to me, is the interconnections that can occur between somewhat isolated places on the map and the fact that NPCs can move between them just like the PCs.
See...told you I didn't know!