This sounds like a good approach. I just ordered Dungeon Delve with an eye to using it for sandboxing, similarly. My current
4e sandbox (converted Vault of Larin Karr) has been going 4 sessions and seems to be working fine.
I think
4e is actually a lot more robust and able to cope with the sandbox approach much better than you'd think from reading the
4e DMG, which advises that if you do run a sandbox (called there a mega-adventure) you constantly tweak the locales' toughness to fit current party level. It also advises that each adventure locale in the sandbox have 8-10 encounters so the PCs level each time. I think those are both poor advice, much better to have smaller locales (ca 3, or 1-5, encounters each), and tweaking them only in extremis. And there should be a noticeable threat gradient, eg in Larin Karr the Underdark beneath the valley is significantly nastier than the overground locales.
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