Just a thought I had while walking from work to a private lesson this evening.
Monster XP in D&D could be factored in such a way that there would be a combat XP total for defeating the monsters, and a non-combat XP total for outwitting or successfully negotiating with the monsters.
Proposed idea for combat XP:
Monster's hit points x maximum possible amount of damage in a round
Proposed idea for non-combat XP:
Monster's Int x Hit Dice
Of course, this is just off the top of my head, and doesn't yet factor in such special attacks as poison or energy drain or paralyzing touches. But it does make dragons and other creatures with similar attacks like storm giants quite choice prizes, as their combat XP would be their hit points squared. For lower level adventurers, low hit die creatures with multiple attacks again offer the biggest rewards, but also the most danger (bears, with their bear hug ability, would give choice XP awards, for example).
And the non-combat XP means that outwitting a T-Rex gets you a little bit of XP, while outwitting a lich gets a lot more XP. Outwitting rewards are small, but could be earned any number of times, as long as the party is able to continue outsmarting their opponents. And for a game where the DM wanted more XP to come from clever thinking than hacking and slashing, they could add a multiplier to the non-combat XP formula (total x10 or whatever), or change it to Int x Hit Points. Of course, the down side (for those without a system that gives monster Int scores) is that Int scores need to be determined for all monsters.
It would be a bit more work for a DM than in Classic D&D, but compared to AD&D, where hit points need to be added to the base value and whatnot, it might actually be easier. The numbers might not scale well, though. Haven't done the math yet. But the idea seems promising.
Being lazy, though, I may institute the second part for my games, since I have the RC which gives an Int score for each creature, and general Int levels are given in the AD&D Monster Manual, but keep the normal awards for combat.