What kind of things can they get in the canteen?

It depends on where they are incarcerated, all commissary lists are different, but similar in the range of available products. Inmates may buy stamps and writing supplies for mailing letters, they can buy snacks and candy or savory foods like tuna, mackerel, salmon, chicken, soups (add hot water), toiletries, grooming aids, dental and hygiene products, sweatshirts and sneakers and various sundries.

I was in county, private and federal prison and by far the most important purchase are your headphones and radio. The currency inside are dollar-equivalent food like tuna or mackerel (sealed in Mylar packets) and in lesser situations Ramen noodle packs.

Here is a link to the FCI-Miami Commissary Order Form - this has one of the better lists of options.