I have found postcards to be a great way to send encouragement, sympathy notes, or thank-you notes.
The small space on the post card for writing means you can usually only fit a few sentences in (keeping the time investment short) and you don't even need to fold up the letter into an envelope. You can start with a few post cards bought from a gift shop, and if you like the habit, you can get themed boxes of 100 postcards from an online retailer for $20. (e.g. birds, or cats, or Disney Princesses).
The small space on the post card for writing means you can usually only fit a few sentences in (keeping the time investment short) and you don't even need to fold up the letter into an envelope. You can start with a few post cards bought from a gift shop, and if you like the habit, you can get themed boxes of 100 postcards from an online retailer for $20. (e.g. birds, or cats, or Disney Princesses).