Why write a letter to your future self?
First of all, it is fun to receive a message from your past self. You will realise time is passing fast and life is finite. Maybe your past self has some lesson to remember or a wish your future would be like.
What to write?
Of course this is up to you but here are ideas for inspiration.
What your life looks like
Describe in precise detail what your day looks like. You wake up in the morning, at what time? Who is lying next to you in bed? Do you go to work? If yes, what transport do you use to go to work? Go into as much detail as you can and you'll be amazed when you read your letter back in the future, how much of it became true.
What goals you have achieved
We overestimate what we can do in the short-term and underestimate what we can do in the long-term. It's likely that you will have achieved more than you write down.
What your life currently looks like
Imagine receiving a letter from 10 years ago with the struggles you were dealing with, or decision you took and are curious how they worked out.
Advice
What advice would you give to your future self? Some mistakes you've made which you should remember to avoid. History has taughts us valuable lessons and we should not forget about them.