Whatever scenario you pick, please be sure to have a clear goal for the PCs, and make sure the characters' skills are relevant to the adventure. Also, the characters should be able to affect the situation in a proactive manner.
I've come across the term "deprotagonization", or leading players to feel their characters' actions cause no change. My worst convention experience had this in spades. We began already infiltrated into a mercenary/ terrorist training camp, posing as recruits. The goal, such as it was, seemed straightforward at first, but proved maddeningly vague. We had to investigate and find the leaders of this group. The referee told us our characters to wait for an NPC ship to arrive. The referee spent at least half an hour describing the tedium of the wait, and the hazing and verbal abuse we received from mercs at the camp. In the interim, nothing we did had any effect. At least half the characters had mostly space skills, with no space ship present. Break into an office? Chat up a low level merc? Try to hack a computer? every attempt was met with, "You can't do that. You have to wait for the ship". Oy.
One of the best scenarios came from two friends of mine. It was D&D. A druid had gone over to the dark side, and had fortified his grove with all sorts of summoned and magically altered creatures. They had a clear goal: go in and kill him. They introduced time pressure, both in the scenario (the druid was going to cast one of those pesky world-ending spells) and real world, with a 3 hour limit to play. They stated the goal, and had a relatively straight path to it. They also included red herrings, such as non-threatening animals, rabbits, bears, etc. and side encounters, including treasure, to lure players off the path. If the players stayed focused on the goal, they had plenty of time to complete the mission. If they dallied, they might run out of time. They also made sure that each PC had at least one encounter their skills were useful for: e.g. traps for the thief, etc.