Four expansions are included in the little box, all of which can be added into the base game separately, or mixed and matched as you like. The Fountain provides 12 new adventure cards and tells the story of the original conquistadors who discovered the 'Fountain of Youth' and who now jealously guard it with their eternal but decaying lives. The Mark consists of six new adventure cards and injects an ancient dark curse into proceedings. The Mountain replaces the base game map with ten new terrain cards and some new rules for a bit of variety. And then there's New Friends - three companions to help you survive your quest.
As usual with Osprey, the production is as high quality as the mechanics, and it genuinely enriches the game play, it's not just an afterthought. The only criticism I've seen is a slight variation on the colour on the backs of the cards in relation to earlier editions of the base game, and a minuscule size difference, only noticeable if you look hard enough. Neither impedes game play. The one tiny criticism I'd have is a design one - the poor kerning of the title on the box cover, which could have done with a little attention.
If you own The Lost Expedition then you really should augment it with The Fountain of Youth - it adds a new dimension of enormous fun and adventure. If you don't own it - go and get a copy! (See my blog post on doing the art for the original game here.)