Depending on where you are, get 50 flowers. Set it up with the wait staff earlier so that when they bring out the cake, they also give her the flowers one by one. If she's shy about her age, then one flower for each decade and she'll know what they represent.

jut keep all things secreat and mangage the music of happy birthday during that time

Give her a hint of where you are going to go.

I'm booking our first holiday abroad together as a surprise for my mum's 50th. How do you think I should surprise her? My only idea is getting a waitress to take a cake to our table with a message iced on it. Anything else I could do?