Go to tour the spa on day 1 and enter the drawing
It is worth going, not a lot of people show up and if they pick a winner and the person is not present they pick another ticket so that gives the people who are actually there a much better chance of winning.
The number who show up depends on how many people read their Navigator! There have been cruises where 6 people showed up, but they only drew for one prize. There have also been cruises where the entire room was full–probably 150 people. I don’t know if the number of prizes is determined in advance or not–I’ve had them draw for one prize and seen them draw 5 prizes.
If you are on a long cruise (more than 7 nights) watch your Navigator. They sometimes have another drawing about mid-cruise!