As a Cities and Knights bonus post, here is a house rule that my friends and I enjoy for Cities and Knights. It actually came from a misread of the rules. Once we learned the actual rules, we decided we still liked our version better.
Cities Walls Protect You From the Robber
City walls are supposed to “protect you from the robber” just by increasing your hand limit by 2. When a 7 is rolled you can have more cards in hand without having to discard.
We like to play that they also prevent the robber from stealing from the city that has the wall. The robber can still land on the hex and shut down that resource. The player that moved him can still steal a card from any player with an unprotected city or settlement next to that hex. However, if a player’s only settlement on that hex has a city wall, that player cannot have a card stolen. It makes the city walls a bit more fun and also gives you a way to discourage people from shutting down your most important resources.