There's a few things that I think made this episode particularly effective for a cultural festival episode:
1. The main cast essentially split up, and each get to do their own things. Due to how different their personalities and roles are, we get to see the cultural festival from four different perspectives. That makes the overall event seem more real and engaging because it takes on the "full life" of a big, special event.
2. This ties into No. 1. In a lot of these "cultural festivals", the main cast ends up having to really "take charge" of it, so you don't get as much of that "simple spectator/minor participant" feeling to it. But here you get a great sense of that spectator feel due to how the Classics Club
aren't stealing the show (at least not yet), and are just doing their little thing, while a couple of their members just take in the festivities and a third has a separate club she has responsibilities for (the manga club).
3. I know many people will point out the quality of the visuals here, and rightly so, but for me it was the
acoustics (i.e. sounds, basically) that hit home so marvelously. The acoustics were totally perfect in this episode, and Oreki's scene alone in the club room demonstrates that well. The way the music comes through into the club room really reflects what these events "sound like" when you're there but trying to get away from it a bit.
This was a very high-energy and just plain fun episode. It's good to see Oreki be actively clever again, even if it is to minimize work for himself.
Mayaka's character was fleshed out nicely here, as this episode nicely displayed some of the insecurities she tries to cope with. Satoshi trying to cheer her up was a nice scene. And Eru's scenes were consistently eye-catching and often very amusing.
I think it was good for the show to take a break from mysteries for awhile, and just let the characters live a bit more naturally like this.