Legally, ANYTHING is acceptable. The classic example of a valid contract is selling a Rolls Royce for a penny. It doesn't have to be an equitable contract...
A print is usually best, except that if something goes wrong and you don't get an image, you can't do that one.
A foreign coin has amusement value, no matter how small its worth: I use to have a supply of kopeks for this very purpose.
If the model signs a release, it is quite hard for them to prove that you did not give them something. This is where something memorable -- a print, a kopek, an unusual cocktail -- is a good idea. Once consideration has been given, and is remembered, the model has to lie, and gratifyingly many people are uncomfortable with this even today.
Generally, if they're friends, I don't bother. A model release can restrict what you can do with a picture, more than the common law does. Besides, a lot of my friends are photographers, and we photograph each other.
The most famous story of a model who was never paid a penny is the short-skirted tennis girl with no knickers on, scratching her bum. To the best of my recollection, which may well be at fault, a young man photographed his girlfriend; there was no model release; and when they split up (as I believe they did), that was the end it...