How can I write my boyfriend a good love letter?

My boyfriend is in a psychiatric program and has very limited access to his phone. Today I got to visit him for the first time in a month and found out that I'm permitted to send him letters/packages. He won't be released for about another 3-4 weeks, and pretty much just sits in his room all day. I want to write him a love letter to brighten his day there at least once, but I don't know where to begin. I don't want to write solely about how much I love when he kisses me or his eyes or whatever, because that's lust, not love. Our relationship has much more substance to it than just sex, but I can't put it into words. I feel as if everything I could say would be an understatement. I care for him endlessly, on a level I've never gone before, but that's an understatement and I don't know how to express my feelings a letter; only with my actions. Help?