My wife and I went around and around about 'how big' our wedding and reception should be. We had a "BIG" wedding - BUT I was behind her all the way watching the budget - and making decisions that spent 1/2 or 1/4 of the way it should.
Here is my suggestion - at our reception instead of buying favors to put on the tables, we had notes printed on 'fancy' paper informing our guests that we had made a donation in their honor to the non-profit wedding preparation retreat organization where we completed out pre-cana requirement. (We are Catholic)
You could make a donation in an undisclosed amount to a charity that you think they would find thoughtful. Here is a suggestion
"Today your dreams come true - and in honor of this day - I have made a donation to the Make-A-Wish Foundation in your honor so that a child's dream may come true as well!"
OR
"I checked your registry and it appears that you have been showered by gifts from family and friends and that you have all you have asked for. So in honor of your wedding - I have made a donation to Habitat for Humanity so that a family in need can get their lives on track!"
Or make a donation in honor of them to the Church where they are exchanging vows (If they are getting married in a Church, can't make that assumption these days - when people get married of roller coasters

)
We all get caught up in giving a gift, but in reality - people today treasure knowing that you thought of them - and thought enough about them to do something -in honor of them - for someone else.
Give what you can - and be socially forward. We live in such a consumption based society - we often forget what is most important.