Messages of affection, apologies for recent arguments, updates about friends and family members, photos of children, or past shared experiences and ideas for future things to share together are all possible things to include in a letter to your husband. The specific contents depend on the reason for the letter, if your husband is nearby, deployed with the military or living far away, the current state of the relationship and when you next plan on seeing him.
If your husband is deployed with the military, make sure anything you send with your letter is allowed under military guidelines. For example, it is not permissible to send food items to recruits in training. Including humor is a good idea if your husband is under stress, and a letter that makes him laugh or recall a funny memory is bound to be appreciated.
Romantic poems and expressions of attraction are also useful to include to your husband, since men love to feel that they are still attractive and desired. Thanking your husband specifically for things you like about him or for things he has done for you are other good ways of making him feel cared for and appreciated. If the marriage is not going well, address the issues in a diplomatic way and try to propose possible solutions that you both can undertake together.