I agree that FC is the best FPS yet (have not played HL2 yet either)
When you are done there are a few options left.
1. Play through again on realistic difficulty. Its a whole new ballgame.
2. Try some of the community built add-on single player levels.
Outpost22 is the best of these in my opinion.
3. Go online and kill other players with a much improved AI (usually) in multiplayer Far Cry.
4. Go mountain climbing. After you kill all the enemies on a level go back and explore the map. There is surprising detail in parts of the map that you are never meant to see and the scenery from the tops of some of the peaks is worth the climb.
5. Try different tactics against the AI. On Boat level, for example, Steal a patrol boat and bombard the shore before you land, climb the mountains behind the camp at the third tower and snipe every last merc before you stroll into the base and take a jeep up to the tower, and there are at least four or five ways to beat the chopper attack by Crow including use of a hummer and without using rockets at all.
Replayability is awesome due to the way the AI works, and there is always more than one way to beat a level from stealth to Rambo and everything in-between.