Yes a card with money would be a perfect gift. You can express your respect through what you write inside of the card. Money will also give the couple a chance to buy something they really need instead of a gift that they might not want or already have.
Money or a Gift Card can often look thoughtless but actually it's a great gift if you're not sure what the couple would like. This way they can go get what they actually like, which is better than just giving them something and they don't end up liking it.
It's an acceptable gift, but not a particularly good one. It would be better to give the couple tickets to a show/sporting event, or a restaurant gift card.
By giving money the couple may believed that you forgot about them or that you were invited at the last minute. Silver is traditionally given for a 25th wedding anniversary, so by placing money in a silver frame, would be your best option.