Characters can only prepare spells once per 24 hour period, not simply every time they rest for 8 hours. Undead live forever, they can just stand around waiting for the PCs to show themselves. Incorporeal undead, anything capable of using illusions, skeletons hidden under loose dirt/piles of bones, etc. makes a decent ambush. Definitely have something set up to attack them as soon as they leave the Rope Trick, preferably one at a time. Make it so they'll have to rest yet again before they continue adventuring, they'll learn soon enough not to Rope Trick in enemy territory.