L.J.
You could make them a memory/video slide show or scrapbook with pictures from their wedding/honeymoon through today. If you want to have some kind of celebration for them, but money is an issue, see if you can find an inexpensive venue like a bank's community room or a subdivision party room & just do punch & cake. Or do both, & show the slide show at the celebration. If they are religious, you may even be able to arrange for someone to come do a renewal of vows for them there as well. Have fun!