Pet Battle your way to 90! That’s right. You can do it in several locations too! Whether you have many or few level 25 pets, you can get some good experience per battle. Especially if you have the Pet Battle trainers unlocked. A few things to remember:
Keep your pets within 3 levels of each other. This way, they all level evenly.
Keep your highest pet within 3-4 levels of the pets you are fighting. This way you get XP for winning the battle.
If you have a 2nd team to swap out, even better. This way you can swap out the full team when your heal is on CD (unless you have lots of bandages).
Where is the best place to go? Hyjal seems good right now. Saturated with pets to fight and the mobs shouldn’t be a problem if you are 85-89.
Wrath zones give you about the same XP per battle, since most of those pets are level 20+
If there is a certain type of pet you are after, you can always farm them while you level and hunt for those rare quality pets to capture.