I'm in the middle of planning my parents' 50th anniversary!
Here's what I'll suggest. Know what they like. My siblings and I wanted to give my parents the moon and the sun for their anniversary. They hated everything we suggested. We had to set aside what we wanted to give them in order to give them something they wanted and could be comfortable with. It's really hard to do when our outlooks are different!
So, what would your parents like? Would they like a fajita truck to come cater a backyard party? Would they like a ballroom at a local hotel with a plated meal? Would they like to just be taken out to dinner by their children or have y'all make them a special dinner at home? How many people do you think you're talking about? 20 or 200?
Do you want to just give them an awesome gift? A portrait of their family or a family scrapbook is the best gift ever, IMO. Make it personal. :)