I arranged for a card from the Queen for my parents Diamond Wedding Anniversary for a laugh really. My Dad always used to say "they don't know what a days work is." Etc etc and when the card arrived it took pride of place in their house and he was showing everyone!
It was like the Queen was suddenly his bestie!!
I arranged a surprise party for them in the local village hall and contacted friends and neighbours and had 3 sisters and a nephew fly over from Ireland and hid them in the hall kitchen and borrowed catering coats and hats for them to come out in with the drinks and my Mums face was a picture!! I got them down there on the pretence that we were just going out for a ride and afternoon tea.
We had a great time and videoed most of it. My Dad died a few years later but it's a great memory.
Regarding the bench, I think that's a lovely idea and maybe a rose bush too. A good garden centre will also arrange for a plaque to be engraved. I ordered a lovely bench when my Dad died and it was about £400-£500 as it was a lovely quality one and included engraving plus taking it to churchyard and concreting it in place.