One of the most unique gifts….I think….would be to give them a pair of mandarin ducks (figures, not the real things.)
In China, a pair of mandarin ducks is a symbol of fidelity and long marriages. You can find mandarin ducks in every possible type of design and substance (cloisonee, wood, resin, jade, porcelain). Some of them are simple and some of them are elaborate. They are also reasonably priced. Just go to Ebay and type in “mandarin ducks feng shui” in the search engine. www.ebay.com You can also find Asian scrolls with paintings of mandarin ducks which are also quite lovely. Depending on the time, if you have two weeks, then you can order from China (they are quite prompt——check the feedback on the vendor). If you are pressed for time, you can find a vendor in the USA that has these. Be sure to check the dimensions as sometimes they ducks may be tiny and sometimes they may be larger.
If there is no time, then consider giving them a flowering tree or a Japanese maple that they can plant in their garden in spring to also commemorate their anniversary.
I was just thinking of something a bit different as a gift.
Hope this helps….!