rope bridges
In real life, I love rope bridges. They used to be all over the himalayas but are rapidly being replaced with nicer bridges made from metal cables. But old rope bridges make the best photos!
The best rope bridges for gaming purposes should be really long (40ft is way too short for any real life rope bridge, they tend to be 3-10 times this long). They should fairly primitive, having sticks instead of planks for footing, and the sticks should be spaced at least a couple of feet apart . In places there should be 3-4 feet gaps where sticks have fallen from the bridge. Some of the remaining sticks should be rotten! A bridge like this can't be crossed by animals, you need to be able to hold on to the side ropes.
I would try to use a rope bridge by staging an encounter when the party is halfway across it. If the people attacking are locals, they don't want to destroy the bridge -- they need it! The people who are on the bridge definitely should have to make balance/acrobatics checks or lose their dex bonus to AC. Also, they will need to have a hand free to hang on to the bridge. If the critters are fliers, they attack those on the bridge -- otherwise they attack those on one end or the other and the ones in the middle have to hurry off (risking falling).
Hope that helps!
Ken