Hi M.,
well, what I did for my parents 40th wedding anniversary was to purchase a nice sterling silver picture frame (8x10) and have it engraved with the date on one side, and on the other, "Happy 40th Anniversary!". My brother and I threw a surprise party (I'm not suggesting you do that!) and we got a very nice picture from the party and put it in the frame for them. I don't think the frame was that expensive - I think you can get stuff like that at the mall at someplace like "Things remembered" or some similar name, or check out your local jewelry store...
Of course, my mom is super sentimental and LOVED it - I am not sure from what you said about them if that would float their boat, but sometimes it is nice to look back and realize that life is pretty good! Or to see how far you have come! Maybe even a nice picture of you and them, or a collage of you and them over the years? If you need more time, you could always send a card with a note that says - gift on it's way...
Anyway, don't know if that appeals to you, but thought I would throw it out there.
Good Luck!
E.