A couple of coins in the patch kit to buy a choc bar or something if you run out of food on those realy long rides is always useful, or in a case of a MTBer that I was doing a epic ride with a good old steak and cheese pie to get him home.
A dollar bill to line a tyre? If I could find one... :)
Yep, a fiver now to line a tyre when things get bad, and watch where you keep your cellphone! I had a mate who punctured his kidney when he came off with his cellphone in his rear pocket of his cycle top!!! :0