Our hotel in London was directly across from a convention center, which would be hosting a huge, multi-day Star Wars Celebration that began the day after we left.
One of our students is a HUGE Star Wars fan and kept trying to make deals or work out scenarios where he could stay behind, attend the celebration, and then join up with our group again later. The best we could offer him was to walk through the center after hours and see if we could get a sneak peek at anything the night before. All we found, however, were a few stormtroopers in their civvies and lots of signs excitedly talking about what he'd be missing.
It didn't help his mood any to know that all the Potterheads got to visit Platform 9 3/4, and the Whovians found their blue police box. But at least he'll have the memories... of missing it by that much!