A little picnic set for two with plastic champagne glasses and a bottle of something bubbly for them to go out and have peaceful moments in the lead up.
An engagement survival kit (google Bridal Survival kit) - you can make your own - stress ball and beer for him, note pad, chocolate, etc for her... my favourite one is here but I like to personalise too:
http://www.bridalsurvival.com.au/shop/for-...uple-c-252.htmlRed balloon for a cooking class together or have one of their pamper hampers delivered as a surprise (you could give them a card saying to make sure to be home for the night on a certain date, then have it delivered that day!
I dunno - our engagement was pretty stressful - it was nice when we got to hang out without wedding pressure!