You could play existing adventures to get some more ideas; this situation was used in several games. As far as i remember, Monkey Island 1-3 would be a great start. In MI 1, you will be a prisoner of the cannibals (and there are several solutions to get out, sadly i don't know the specific riddles). The second game features a prison cell and you had to work together with a dog to get the key (lure him with a bone). The third part of the series has a nice situation: you are trapped inside a crypt, and have to convince the caretaker that you are a mighty demon and he should open the crypt (you have to use a speaking skull, a lantern to produce freightening shadows etc.).
EDIT: ok, nice idea: you don't need the game to get the riddles - just use the free availables solution
I looked up the solutions for Monkey Island, so here are some parts from dlh.net:
Monkey Island 1, being trapped in a hut of the cannibals:
Inside the hut, get
the skull on the floor to reveal a loose board. Get the loose board. Go down
the hole to exit the village.
After the cutscene, Guybrush will
be in a cell. Get the rock hard mattress. Get the stick. Use the stick with the
bone on the left. Get the bone. Give the bone to Walt, and he will drop a key.
Get the small key. Use the small key in the door.
You'll wind up in the Goodsoup crypt.
Walk south from the tomb and past the widow's ghost. You'll overhear Murray
plotting to scare Guybrush again. Take the crowbar out of the coffin and then
walk over to the crack in the wall. Murray will drop down from the ceiling in
attempt to scare Guybrush. Pick him up and then look through the crack.
Use the skeleton arm on the lantern and then use Murray with the lantern. The
crypt will be opened, allowing you to leave.