Second on the throw away clothes for bike trips. I have a whole spacebag full of ratty jeans, t-shirts etc.
A fairly decent mattress is a $3.00 float from Wal-Mart, they work OK, pack well and are cheaply replaced when needed.
If taking food - soft taco size tortillas double as bread and they are already flat! Country ham biscuit slices need no refrigeration, is very light and thin. Condiment packs are GREAT! I take a Coleman Sportster stove, mess kit, percolator, can of Sterno when camping. I cook and make coffee on stove then keep it warm with the Sterno while still working with stove. For the first night I had an excellent dinner of canned chunk chicken spiced with Taco Bell sauce packs made into enchiladas, Ramen noodles and cold beer.
Sunscreen and bug spray (if camping)
Packable fishing rod if near trout streams, lakes etc.
Extra key if you have it (I threw mine away once in my ratty throw away jeans)