The first quest with goblins in a mine could have the goblin boss summon a Magmin (MM apge 213).

Put a cave somewhere on the way. When they go to find a place to sleep and roll a perception check when looking for one, the one with the highest roll will notice theres a cave thats hidden. Inside there will be a crawlspace where only a single person will be able to squeeze through. There will be a . Behind the door an assortment of thing is hidden. Quite a number of various magical items. Most of them have useless enchantments.