Figuring out what to do with experience

Now that I got enemies and combat implemented, it's a bit hard to figure out a good way to spend those experience points. I want to stay away from a skill tree since it would be complicated. Increasing just strength or defense might get boring. An other option could be to increase proficiency at fighting a specific enemy (ex: if the player kill 100 rabbit, then it will be easier to kill rabbits).

No as easy as I expected.