I don't always run OD&D sandboxes, but when I do, I use my sandbox toolkit. Today I invite you to peer behind the DM screen.
The Massive Binder contains:
Compiled Judges Guild Wilderlands tables.
Cities by Chaosium, my go-to resource for town/city creation and random urban encounters.
OD&D reformatted; much easier than breaking out my LBBs; still the best system ever conceived for improvised play. Iron Falcon may soon graduate to this spot.
Plenty of hex and graph paper.
And extra no-frills, lo-fi character sheets.
I also keep the Judges Guild Ready Ref Sheets (so buttery) and the 1e DMG at my table regardless of edition.
I have plenty of other options in my bag of tricks, but these are the "core". With these tools, even spending five minutes on game prep seems....uncivilized.
Go ahead PCs, make my day.