The old Book of Lairs (I and II) are great for this. Cheap too (in PDF).
Thank you! These are very useful.L1 Secret of Bonehill: the keep and dungeon at bone hill can easily be dropped in a sandbox as its very self contained.
UK2 The Sentinel: Has 3 locations that you can scatter on a map; bandit camp , ruined villa, druid cave, that player can find or explore. No need to include the macguffin hunt as a pretext.