One thing that always put fear into the players at low through mid-levels was random encounters in the wilderness because they never knew what they were going to encounter. They knew they always had the option to run. One time, at 6th-level, the players ran into an old green dragon in the forest. I gave the players the benefit of being able to notice the dragon before the dragon saw them (or actually, before the dragon decided to do something since it knew the players were there anyway). The more experienced players ran away. The less-experienced (one guy) decided to hang around and watch. The dragon started walking forward to scare him away but he stood his ground. He ended up being an afternoon snack. Now the player knows what the better choice is next time.