something nice to do for my parent's on their anniversary tomorrow?

Why not make them dinner for two with candle light at home? Then you go out with your siblings and give them some alone time...

I think a gift card is fine. You could also pull a poem off the web about marriage or anniversary related. Print it. Get a cheap decent sized frame. Put the poem in the center. And decorate the edges with pictures of them through the years. That is something really cheap but from the heart. Im sure they will appreciate anything you give them though.

Make them a nice breakfast or dinner or clean up around the house. Just remembering will make them happy.

You should put together a homemade card with pictures of you and them. Writings of how much you love them and what they taught you. That's sounds pretty cheap but the rents will love it!

Listen, dont say no, clean the house, cook them dinner, then do the dishes

how about baking them a cake?

something that you make from the heart is so much better than just buying them something.

how about a hotel room for them and then u can go have a big beer party at the house