This is another post that'll be short. This is because, much to the chagrin of my OSR brethren, I'm never good at placing traps and coming up with puzzles. I can execute ones that others have devised (thank you, Grimtooth and KoDT!), but never have I excelled at creating them. Heresy of all heresies, let the find traps or INT roll do the player's work!
That said, one I like to use sparingly so it's always a surprise to players (and I'm not sure if it's a trap or just an encounter/obstacle) is to place leather-wearing, wicker-shield-bearing, wooden-spear-wielding kobolds that have rust monsters as pets! Combined with rope traps and deadfalls (you do know what the Vietcong put on punji sticks, right?), this can make even a mid-level party take note of the little dog-faced pests!
Until tomorrow, gentle readers, oil your weapons and armor!