Creative handicaps in Settlers of Catan?
The best handicapping is to require the more skilled player to have no 6's nor 8's in their chosen starting spots; a step harder is to also block the 5's and 9's, but that often results in not being able to find a spot to place starting bits.

Commensurately, the biggest problem for casual players is NOT picking at least one common number spot...



An additional option is to restrict the number of cities and settlements for the experienced players. Reducing one each still makes the 10 point win possible ((3x2)+4), but only just.