This is just my opinion, but 2 challenges are really easy that I know of, but they are not worth a lot of XP.

Goodbye Challenge: Fall 30 or more feet to your death - 500 XP

Base Jump Challenge: Fall 15 feet or more & survive - 750 XP

Other than that the best way to gain a lot of XP (but not very easy) would be to kill enemies using one weapon. If you kill 1000 enemies with one weapon, its 10,000 XP. Once you finish the 1000 kills with one weapon, switch to a new one. Same goes with the perk challenges.