A fun variant is trying to score zero points without penalties. So no beaching boats on an island, leaving houses in the sea, etc. Not as easy as it sounds!
I like this.
The only problem is you can't play boats because they always score at least 1 point in the sea. If it doesn't mess up the balance, a better rule for boats might be "1 pt for each square away ... minus 1 pt" like forests. Maybe with a bonus point for each square beyond say 5.