I have a secret box & when I find myself with some extra money I tuck it away. I try to put away a few quarters or dollars from my wallet every few days. I add "bonus money" in too - money you find in the laundry or a winning from a scratch lotto ticket. I never miss the money & it adds up.
I don't have anything specific planned for this money but it makes me feel good knowing that if something came up in the future I can say "hey, I have an extra $150 let's do it!" In fact I get really greedy on what I want to spend it on because saving becomes addictive!