Bonus XP Suggestion #1: Lets say you're fighting npcs and manage to kill everything without taking a hit. I think there should be bonus XP for managing to do this. A 10% bonus if you killed everything in turn-based without losing a HP, and 20% for killing everything in real-time. (Real-time is usually more difficult, but the bonuses could remain the same for both!)
Bonus XP Suggestion #2: Lets say you successfully detect an encounter with a player in it, I think you should be given a small amount of bonus xp for each player detected, and a large amount of xp for detecting a car! Of course detecting the same player again on the same in-game day shouldn't give you bonus xp for repeat encounters. (There could be a cooldown for cars but that might be overkill)
Bonus XP Suggestion #3: Experience for killing a player! This would depend on what the other player's level is. If they are much lower than you - you'd get very little, or even no xp for killing them, however a player close to or equal to your level you would get a nice amount of xp for killing them. Killing the same person again on the same in-game day however would net you a 'You feel no glory in killing this person' message or something like that, also killing someone in the same faction would net you a similar message, such as 'You feel no glory in murdering your comrade'.
Having people get no xp for killing people level 10 and below would prevent people creating alts for a single character to kill over and over again, also kills on people a higher level than you would just be counted as a 'same level' kill to prevent exploitation.
PS: Sorry for posting so many suggestions so rapidly, i've been bursting with ideas recently.