In general, rolling spot and listen checks work well with messing with your players.

In my case, the players were proceeding up a corridor with a number of rooms in it, each with big pipes and some unimportant cleaning supplies in each. I asked them if they wanted to search each room, and who's going first.

The first two rooms the fighter went first, a bit cautiously, but only finding the cleaning materials and odd noises through the piping, like laughter.

The party never managed to reach the third room, because they all failed the check to spot the Gelatinous Cube that was in the way. They didn't even realise it when I asked them to roll spot the third time until BAM, face full of Cube.

They actually enjoyed that encounter a great deal.