I recently have found a really fun thing you can do to other civs that share a penninsula with you. In the expansion era send a scout out to see what kind of land you are on. If you are on a penninsula and there is a relatively narrow section (1-4 tiles) between the penninsula and main continent (if there is one) you can easily stop your rivals from expanding beyond you by fortifying a spearman there. Easy, safe, and hilariously entertaining to see how easily you excel past your oponents...
-RC
-RC