Depends how much and what they like. We just celebrated it and didn't want ornaments/clothes etc so daughter got us a day at a health spa (swimming, massages, etc) which was fab. Vouchers for short break or meal? I think the old custom of giving ornaments etc is dying out now as couples have a much younger outlook than in the past and we're seeing more and more make it to 60/70 years wed and more!
I gave my parents a bottle of vintage ruby port and 6 bottles of red wine. If your parents are not drinkers then perhaps a treat of a weekend break in a hotel.
My sister came across a fantastic website that turns children's drawings into jewellery and other things in silver. For our parents 30th wedding anniversary we got them a silver plaqu that had a drawing from both my sister and I that we did when we were very young! It was a cat and a horse - they were delighted as it was so personal. The company is AM jewellery and the website is www.amjewellery.co.uk My mum is always showing it off when people come to the house! They also make jewellery and I was thinking it would be nice to have a necklace or brooch made when my 2 children are a bit older.
My wife & I bought her parents a very nice ruby-coloured RHUS BUSH for their garden. They were delighted with it and tended it for very many years as it flourished. I think we gave them a bottle of bubbly to open on their big day, too.