I've been to a slew of weddings recently and thus have had to give my fair share of gifts celebrating matrimonial bliss.
Contrary to most other gift-giving occasions, weddings are NOT the time to get super creative. Generally, it is advised to go off the couple's gift registries. Should they not be registered, money is also appreciated. Other times, the couple may suggest a favorite non-profit organization so that donations can be made instead of receiving gifts/money.
Although the main gift should follow the general rules above, you can always add a personal touch! Go ahead and put a cute picture of the couple in the sterling silver frame you purchased off their registry, or add an inside joke to the card, or even knit them a trivit in their favorite colors. Touches like this make even the most mundane of gifts somewhat personal and ensures that the couple is still receiving what they want!