Levying- Until I played a game as Hungary, I'd never done it. Lately, I've started doing it more though. It's 2 free era score, it can be relatively cheap depending on the circumstances, and best of all my most evil use: When playing with Apocolypse mode activated, you can sacrifice levied troops to volcanos instead of having to sacrifice your own.
National Parks- Not invisible per se but I never use them anymore. Rock Bands seem to always be a better way to convert faith to tourism now.
Giant Death Robots- If I'm going for a war game, I've already secured victory by the time I can build them. If I'm going for any other victory, they're just a waste of time.
Tunnels and Canals- I think I have only built 2 tunnels ever. Canals I always wish to build but the map generator never wants to give me a good reason to.
Various specialized tile improvements- I don't think I've ever built Liang's City Parks, maybe I'm missing out. Forts, Airstrips, Missile Silos and Railroads are all so niche and it never seems worth investing in Military Engineers to build them to me. I suppose I could see it in Multiplayer maybe? There's also some city state unique improvements that are just hard to justify using build charges on, though this is far better than it used to be.