Buy a cheap box of chocolates and put the candy aside.
Inside the box, put together a box of pictures that are of the two of you. On the back of each picture, write a thought about what the picture means to you personally. Does it elicit a specific memory? Then detail out as best you can, what that is.Don’t just settle for the details of who is in the shot and where it was taken, try harder than that. If you need to, ask someone to do the physical printing for you, that way it’s still a surprise for your partner.
You can also print it off a computer and type/write your sentiment on the bottom of the page. Or make the picture an attachment and write your memory out that way you can email it to them. Keep adding to the box as time allows. Let them add theirs too or even add their thoughts to the pictures you’ve written about. This is a great gift for a long time to come as sometimes the chance to tell how you feel about a good memory can get lost in an instant.