I know they said no presents, but a really nice engagement present is a silver cake slice which they can then use at the wedding. I have heard of people getting them for engagement, and then using them at the wedding and first birthday parties for their children.
Actually I second that. It's great if you engrave it with the date too, then later they add the dates of their wedding, and children's birthdays etc. Friends of mine did that and it's definitely a gift that gets used a lot