Modules I've used recently for sandboxing:
My Classic D&D Campaign
Pretty well everything from Basicfantasy.org, notably Monkey Isle, plus tons of other great adventures.
X1 Isle of Dread
B7 Horror on the Hill (much tougher than X1!)
DCC The Secret of Smugglers' Cove
My 5e campaign
Dyson's Delves I (just bought II) - the fully developed adventure sites are great, highly recommend
Caverns of Thracia - used the 3.5 version
Liberation of the Demon Slayer - a weird one
Halls of Tizun Thane & The Lichway from Best of White Dwarf Scenarios I, both by Albie Fiore.
If you want tons of great free modules for sandboxing go to https://rpgcharacters.wordpress.com/ for Dyson Logos and http://basicfantasy.org/downloads.html for the Basic Fantasy ones. For print go to Lulu.com for Dyson's Delves, go to amazon(!) for incredibly cheap at-cost printings of the Basic Fantasy works.