Honestly...? Almost every single tabletop game I own and play, except 2-player Kosmos games or something like Settlers or Stone Age or maybe Dominion.
Trading card games...
Pretty much most of the music I listen to isn't exactly easy listening or default stuff, so I can relate to the OP. I probably would have given them a mix of Dream Theater songs, but it would have been tough to compile a mix that would appeal to the average listener, but a few of their tracks are definitely the most "accessible" Prog Metal I've heard, as opposed to say, Symphony X or something, but I digress. I've tried getting a few friends into Instrumental Rock, and some of them think it's awesome, others say "so it's like a 5 minute guitar solo wankfest? no thanks."
I would list most of the games I loved from the mid-90s here, like X-Com or Jagged Alliance - I can see why plenty of people wouldn't like turn-based strategy, or even understand why I like it.
PC gaming in general has a stigma to it, but I've gotten friends into some sweet indie games by getting them humble bundles for Christmas and such. It's tough to really "recommend" someone to get into enthusiast-level PC gaming though, especially if they already have a 360 or PS3, but some of them come over and I'm playing something really different and out there compared to what they're used to, and they get interested. It's like the first time you see the Tactical Zoom in Supreme Commander, if you only remember RTS games like Starcraft 1 or Warcraft 2, or seeing someone flying around at 200 km/h in Tribes when you're used to slower-paced modern military shooters (Note I am not using the word "Realistic").
Obviously roguelikes are something that isn't easy to recommend to everyone, but some people appreciate the depth and complexity.
So those are a few things I can't really "recommend" to everyone, just like my love for CRPGs, JRPGs, arena FPS games, tactical FPS games, etc, don't appeal to everyone (people say Quake Live is "mindless" when it actually probably has the most depth of any e-sport on the market at the moment).