Trick House was definitely fun. I never used to figure out what I had to do, even when I did see the sparkle, which was frustrating, but once I discovered Trick Master for the first time, I was hooked. I mean like, literally going back to it every time I caught a pokémon, beat a gym...anything. Just in case there was another challenge.
I always thought it would be good if there was a post-game challenge with all the gym leaders in for single battles (as well as having their double-battle rematches in Emerald).