I got married back before computer's were so widely used and my mom always gave to my friend's a bride box to keep her addresses and gift records in.
But since all that seems on computer now - what about a wedding scrapbook for her to start saving all her pre-wedding memories. Other ideas are toasting glasses, a bottle of champagne, cake cutter set or guest book.
From the article below - I like the idea of a wedding countdown clock - that would be fun and unique!
Found this article online with etiquette and ideas:
When a couple makes the big decision to wed, an engagement party is often given in their honor. While etiquette doesn't require you to bring a gift to the party, you may choose to share in the soon-to-be-wed couple's joy by honoring the occasion with an engagement gift. Even if no formal celebration is held before the big day, you might enjoy taking this opportunity to give the couple something practical, fun, or perhaps even sentimental.
A great engagement gift can be something useful for the home, such as a high-tech wine bottle opener, luxuriously scented candles, or a subscription to a gourmet cooking magazine. Alternatively, you could choose a gift in keeping with the celebratory spirit of the occasion, such as a bottle of champagne, a boxed set of romantic music, or a gift basket filled with relaxing spa goodies for their honeymoon. You could also give something to help the happy couple prepare for their big day, such as a wedding planner journal, wedding survival kit or even a wedding countdown clock. If you're very close to the couple, you might even give them an item that can be used as part of their wedding celebration, such as a guest book, a personalized cake server, or a framed picture of the happy pair to place on the gift table.