Remember that how you give is as important as what you give: Unless you know gift-opening will be on the party agenda, send your gift ahead of time. This not only saves your hosts the trouble of managing presents during the festivities, it also prevents embarrassment on the part of attendees who didn't bring an anniversary gift, and awkwardness for the couple (who would have to acknowledge your gift in front of other guests).
For gift ideas, one source of inspiration is the traditional list of anniversary gift themes. In general, only the immediate family gives expensive gifts, so even for the list's milestone-anniversary recommendations (such as silver for the 25th and gold for the 50th), a modest gift such as a silver-plated photo frame or gilt-edged scrapbook is appropriate.
Many couples will appreciate a bottle of champagne or wine. For a personal touch, give the same as that served at their wedding reception. A gift certificate to a top restaurant or tickets to a play, concert or symphony performance also makes a thoughtful and romantic anniversary gift idea.
The couple's shared hobbies can provide more ideas. For example, consider an experiential gift certificate for an activity that they've always wanted to try, such as a hot-air balloon ride or dance lessons. If they're adventurous cooks, they might enjoy a cookbook and gift basket of ingredients for an exotic new cuisine. Or, if they love outdoor activities they might appreciate new sporting equipment or a selection of gardening accessories that they can enjoy together.
Take your knowledge of the recipients to Gift.com's GiftFinder for help finding a personal and inspired anniversary gift that the couple will treasure. You can search for anniversary gifts here.