DM "smart sword" trick

Your players are going to do what they're going to do and its hard to put them in the right place at the right time for something you planned on happening. To not give them choices means they're on a "rail" system of gaming, which is boring.

Example: characters come across a huge door. I want to use it later for something but the characters are bound and determined to figure out some way to get through the door. I want to tell them "wait a second" but I have no way of doing that without giving something away. We waste like 20 minutes and the characters are frustrated nothing would open this door I've introduced. Frustrating.

What do you do?

Solution: A "smart" item. The item can either see into the immediate future or has advanced senses like heat vision, scry, detect alignment. Whatever powers I give it, in the game its really just a way to pass the characters hints about what to do without being overly obvious. How is it that this section of the dungeon has never been discovered before? Nobody had a sword like yours.

This trick has more than one use: imagine the characters have just encountered an enemy that is far more powerul than they. Their most recent encounters have been successful and they're feeling like a good fight. Realistically if they take on this enemy, they're going to get pummeled. I pass the characters a note explaining the sword is self-lowering itself. The characters get the message and try diplomacy instead without having to pay out big time in healing potions and magic with no return.