In a sense, the title says it all, but if you want some thoughts…
Write a “letter from the heart.” You could write to someone from your present or past. They could be someone you see often, or someone you may never see again. They could be living or passed away. You could even write to someone you never met.
This could be a letter of gratitude, or it could be a letter where you address issues you have with this person.
NOTE: You will NOT necessarily send this letter. The idea here is to explore your ideas as deeply as you can. This is, basically, a structured free-writing exercise, so you need to feel that you can go wherever your writing takes you. In fact, if you write a letter that is particularly deep or emotional, wait a few weeks before deciding to send this letter at all – give yourself time to reconsider what you’ve written before you commit to sending it. This assignment is borrowed from the book “Don’t Sweat the Small Stuff” (which I highly recommend) and it can be very effective.