Cannister foams are very simple once you get the basics down.
First, know when enough is enough... when you go to empty the cannister, make sure all the gas is discharged before you unscrew the cap. Once you master this basic principle, hand it over to a novice to empty and clean, now we're talking foam.
Anyway, almost anything can be charged. I would recommend starting with cream based foams in order to aquire an opperational platform to expand from.
For most flavor infused or blended creams, dissolve 1 to 1 1/2 sheets of bloomed gelatin per cup of liquid. Always chinois or tammi anything you funnel into a cannister. [2 1/2 cups of cream = 2 1/2 sheets].
For a starter, try a thyme infused cream, drop some thyme into some heavy whipping cream, simmer, steep, salt, add your bloomed gelatin sheets that you graciously barter from pastry, strain into your cannister, ice bath it to a safe level, cover, charge and you're ready to go. Notice that it take some time to set-up before it becomes stable. Ice bath if you have to, but try and allow 6-12 hours.
Recommended rule, after each charge of nitrous that you add, vigorously shake upside down [the cannister, that is] 10 x, prior to discharge, shake firmly 2-3 x.
If you can't feel the contents moving inside during the shakes, don't be afraid of running it under HOT water to loosen or ease up the "set".
Before you know it, you'll be experimenting with who knows what.
When you're ready for purees or hot foams or personal indiscretions, get back to me. :bounce:
Be sure to check out ISI's website and there's alot of recipes out there, Rick Tramonto's books are a good place to start.