Creative ideas for including RSVP in wedding invites?

In coming months I am going to be working on wedding invites for a friend. I'm currently collecting ideas so I can show her some samples, but I am not seeing much that details how to include the RSVP cards.

My biggest worry with all this is making sure the invites stay under weight so they can be mailed with a single stamp.

Any suggestions will be greatly appreciated. Thanks!

Using Vellum as a layer add a lot but is very light weight. Brads and buttons can be bulky. Twine is light weight embellie. The candy dots (Stampin' Up) are a good brad alternative- they are flatter and lightweight- good for mailing.

You can even do RSVP as postcards which would reduce envies and postage! Those are just some quick thoughts.

I just added this simple wedding card to my gallery at Splitcoast Stampers for an idea?

http://www.splitcoaststampers.com/gallery/showgallery.php?cat=500&ppuser=29995

One of the great ideas that I have seen/received for the RSVP is to include a wedding dress and a tux jacket loosely inside the invitation. On each is the RSVP if you are for the bride or groom's side. These could be printed on a little lighter weight of card stock than the invitation to keep the weight down for postage.