- Choose religions (for flavour)
- Random seed on reload (so in case I reload a previous save to try a different strategy, not everything is predictable)
- Random personalities
I've tried no tech brokering, no tech trading, and aggressive AI and find that each one tends to give more of an advantage to the human player at considerable expense to the AI, at least in the long run. Aggressive AI especially helps the AI at first, but by mid- to late game, supporting all those extra (often unneccessary) units puts the AI way behind you in techs. The AI also doesn't take good advantage of advanced start (though it's a boon to get through the first few otherwise-boring turns on Marathon speed, so I sometimes use it).
Of all the custom settings, I find that Random Personalities is the one (besides the difficulty level itself) that introduces the biggest challenge to the human player without adversely affecting the AI. You have to watch your diplomacy carefully and don't know who has Monty's/Shaka's personality and who has Mansa's/Gandhi's until well into the game, if at all. The AI, on the other hand, bases its diplomacy entirely on what happens in-game rather than on pre-determined notions, so it adjusts to RP perfectly well.