If you search in the galleries for "engagement" you'll find HUNDREDS (thousands, maybe) of engagement party cakes.
You could do a wedding bell, a big engagement ring, two wedding rings, a heart with the couple's names. Those are just a few off the top of my head.
Be sure to post your cake when it's done!
For a more personal touch you could ask the couple about their engagement story. Maybe there was a specific food/restaraunt/location, etc. that you could incorporate in the design of the cake? Other than that, were there any specific requests like color, shape or size?
Here are a couple ideas from my photos.
I hope these links work...I'm not too hot at linking pictures etc. yet! If not, they are the 'strawberries and roses' cakes in my album.
http://www.cakecentral.com/cake-photo_41412.html
http://www.cakecentral.com/cake-photo_47648.html
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