I have often wished they had jewelry for men like they do for women representing the 'past, present, future' type thing.

Perhaps something that would be for the both of you? A special weekend trip that would include representations of your life together? Starting with the dating, serious lust, living together, marriage, first house, and ending with time including the kids? Kinda like "This is Our Life"? May take more planning than what time you have, but may give you both something to reminisce about.

Plant a tree and place a 2 seat bench nearby so when you 2 need to just get away to sit, talk, whatever you have a place?

Or, get together and plant a time capsule to dig up when you have your 40th?

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A blessed thing it is for any man or woman to have a friend, one human soul whom we can trust utterly, who knows the best and worst of us, and who loves us in spite of all our faults.