The official material is extremely easy to use at the table. It is well organised and terse. The critisism could be that is offers only the outline of an adventure. But for me it works great. I would recommend:
Halls of the Blood king
The Hole in the Oak
The Incandescent Grottoes
Winter's...
I once ran a campaign with old modules. For conversion just adapt the statblocks. Goodman games also has solid 5e conversions. And they always add some additional material that is great for use in a sandbox.
I can recommend:
B2 Keep on the Borderlands (OAR 1 Goodman games)
T1 The Moathouse (OAR...