Most people who have been married for 50 years have everything they need and then some... we just did my parents 50th and we asked family and friends to send pictures of them self and our parents from past and present, and asked them to write a small note with it and we put it together for them in a gold album. we put it on a table were all the guest could look through it. it was a great hit with everyone.
To be frank I think a table would be more proper, talking about keeping it as a memory. You see a money tree might dry up or something but the table can always be there to give back memories.