There aren't many things I can't live without, but this is one thing my family and I use everyday.
Enter the Wysiwipe. Tiny, enivironmentally-friendly and biodegradable towelettes. They look like small tablets, hardly bigger than a coin, and they don't weigh a thing. All you have to do is add a small amount of water and they expand. All of sudden I have a wet towelette with which to wipe oil from my bike off my fingers or wipe my baby daughter's face after she massacres a piece of melon.
No perfume, no toxins, nada. Just a biodegrable towelette that you can discard with a clean conscience.
I always carry some Wysiwipes in my bag and, since I usually have a bottle of water in there somewhere, it's the perfect combination. Perfect for urban cyling or any outdoor activity you can think of.
Available for purchase online at the Wysiwipe website. You can even see a film about how they work.
Ah, life's simple pleasures... :-)