Don't design multiple solutions to the problem, but design multiple methods of finding the one right solution.
If you are having trouble coming up with multiple solutions to a problem, or have designed a problem that rightfully should only have one solution, a good alternative is to incorporate multiple methods of finding that solution.
Opening a locked door with multiple solutions:
Being a simple door, I am sure you can think of a myriad of ways to get through it.
- Use a key (that is, if you have obtained one).
- Pick the lock.
- Try a way around it.
- Break down the door.
Opening a locked door with a single solution, multiple paths:
The only way to open the locked door is with a key. You can't break it down, because it's magically reinforced. You can't dispel it because it was reinforced by a high level archmage. Basically, the key is your only option.
I can improve my answer if you'd give me an example of an encounter for whichBesides finding where the key is first, obviously, you insist there couldwill have several methods of obtaining the key:
- Find the key-bearer, and persuade him diplomatically.
- Find the key-bearer, and pickpocket him.
- Find the key-bearer, slay him, take the key.
- Find the key-bearer, drag him to the door, ask / force him to open it for you.
Your obstacle would effectively have only be one possible solution: a key that unlocks the door. But you'd be able to take multiple paths to obtaining this key.