Fluxx

 

Nomic is awfully abstract to wrap your head around if you haven't actually tried it. One option is to join one anyway and just see how it goes. Another is to look at Fluxx, a much simpler game that still lets you play around with having the rules change out from under you in the middle of the game.

Fluxx starts with one Basic Rule ("Draw 1, Play 1") and no way to win (yet), and has four basic types of cards:

1. Keepers. When you play them, they stay in front of you until some card says otherwise. ("Milk", "Cookies", "The Sun", "The Moon", "Television".)
   
2. Actions. When you play them, you do what they say, then they get discarded. ("Steal a Keeper". "Take Another Turn." "Rules Reset".)
3. Goals. When you play them, they become the way to win. ("Milk and Cookies": If you get those two Keepers in front of you - or if you already had them in front of you - then you win.) Any previous Goal is discarded. (Oh, you already had one of the Keepers for the previous Goal? Oops, sorry about that!)
   
4. New Rules. When you play them, they affect everyone until some card says otherwise. ("Draw 4", which overrides the first part of the Basic Rule. "Play 2", which overrides the second part. "Keeper Limit 3": if you have more than 3 Keepers, you have to discard some. "Hand Limit 3": if you have more than 3 cards in your hand, you have to discard some.) Any contradictory New Rule already in play is discarded.
   

There are many themed variations (Star Fluxx, Stoner Fluxx, Monty Python Fluxx) that follow this same pattern. Some of them add new types of cards, like Creepers, which are basically anti-Keepers (they prevent you from winning, unless the goal requires them, or some card says otherwise).

There's also a Fluxx board game, where the Keepers are on the board and you have tokens to move around and occupy them. In addition to the usual shenanigans, you can rearrange parts of the board, change how many spaces you move each turn, and change how many goals you need to complete to win.