You can make a 4-button combination lock using a demon (or other monster) like so:

It works like this. The Demon starts out in the first room (woken by the players gunfire ofc), with 4 teleporters behind “S1-Door-Open-Stay-Fast” doors. The switches are available to the player in the main part of the map, and eah corresponds to a door blocking the demon from the teleporters. The red teleporters in this illustration represent incorrect switch throws - the door opens up and the Demon teleports elsewhere aborting the combination. The green teleporter represents a correct throw - the Demon teleports to the next location in the sequence.
The next location has 3 doors for the 3 remaining switches in your combination. The correct throw teleports the Demon to the next location in the sequence - 2 doors for the last 2 switches. Then to the final location for the final switch.
The final switch opens the final door. The demon runs across a linedef (the dotted line) that triggers a “W1-Door-Open-Wait-Close” door. This door is timed and will never open again - BUT - you can add another teleport to the final room that teleports the demon to a hidden area where he james the door open. This part is hard to explain, but the example wad here should show it well enough:

ccccombo.zip
(use doom2.wad)

You could theoretically add more buttons, but I think it would get more cumbersom and less likely to run smoothly. As is you would have a hard time breaking the sequence, even with the switches directly next to eachother, and would probably have to rely on tool-assist to do it. The longer distance the demon has to travel to make it to the teleporters, the easier you could break the sequence.
If you want to be able to 100% the kills in the map you'll need to add a crusher activated by the player at the end of the map that smooshes the demon. So he'll have to be straddling 3 sectors (this will make sense if you look at the included wad).
Results:

That's it. I think you could use a similar set up for other things. Obviously elevator activation would be one… I dunno. Use your imagination.