Don't know if anything like this is recommended here but here's a variant which gives combat more incentive. After defeating a monster look at its health and roll a die if you roll within that no. you get a random rune. Eg you kill a skeleton with health 2 on a 1-2 you get the rune. It works well as you are more likely to get a rune the more powerful the enemy and it gives some purpose to encountering the enemy.