We did an article in S&P about exactly that years ago! From memory...
Gladiator is worth a look if your players are going anywhere near an arena.
The Book of Strongholds & Dynasties will serve well in higher level play.
Ultimate Equipment/More Ultimate Equipment has some interesting items that translate well.
OGL Ancients can work well in some kingdoms.
Slayer's Guide to Amazons could be interesting in a deep jungle.
The Book of the Sea if you want to expand out Pirate Isles.
Things like the Slayer's Guides to Scorpionfolk, Yuan-ti and Titans are more limited, but could work well for Conan in one-off deep wilderness areas...
