If I just want the happiness spike ASAP, then I would just buy $15 worth of Magic cards, or maybe a booster pack or three, depending on the set.
If I'm trying to maximize my overall happiness over any amount of time, I think spending $15 on something reusable, like a nice pair of socks or a hat would work out for me.
If I want to maximize happiness in general, then spending it on someone else I care about does double duty. It makes someone else happy, and I get happiness from being involved in the process.
Darn Tough socks on sale. Merino wool, so you dont have to wash them every time if you use wool wash and air then out (and don't wear then without shoes (dirt)), and a lifetime warranty. Now that's a nice $15 pair of socks.