ID and undies, duh.  The rest is optional.  Try to keep it light so you don’t accidentally lose anything or have stuff get lost in the bag shuffle.  Oh, and try to plan where you’ll keep your ID/phone/etc. while you’re out running the streets.  A baggie into your undies?  Gross, but it works.  Stuffing into your socks?  That works.  Fanny packs are always hilarious, too.