This puzzle room is a great addition to an adventure map. The player has to shoot the wooden button to create a platform above the deadly lava in order to obtain the prize shot from the dispenser.
Step 1: The Base
Make a 5x11 hollow base. Add sticky pistons to the left, inside.
Step 2: Making Up the "Corridor"
Place Stone Bricks (or whatever material you've been using) in front of the pistons.
Then, add 2 blocks above the blocks on the right side to start making up the room, also place one block at the very back above the block behind the pit. Next, add a wooden button (NOT STONE!) on that block and a dispenser above (like in the image)
Step 3: The Lava Pit and Redstone
(You can now fill in the pit with lava)
Place Redstone Dust starting from the block with the button and around the pistons.
Step 4: Finishing the Room
Cover up the partially made room like in the image. Making a 4 block high room starting from above the pit.
Step 5: Details and Changing the Lava Pit
You can then add details like I did with extending the front of the room and adding a door and a torch inside above the dispenser
Optional: You can make the lava pit deeper so that when the pistons push the blocks, the lava won't disappear. (you will see why this is important later on...)
Step 6: Testing and More
Add a prize to the dispenser (don't use splash potions, as they will reach to the player)
Test the contraption by shooting the button with a Bow and Arrow, if successful, the pistons will push the blocks and make a floor for players to reach the item dispensed.
Step 7: The Trap
If the victim tries to retrieve back their arrow, the pistons would retract and the player would fall in the pit and die before they even reach the door...unless they're pretty quick, in this case it might be better to make the room longer :)
This is also the part where making the pit deeper is important, if you decide to, the player will fall into lava, if not it will just give them a scare, and they can still jump up from the 1 block pit...