Had fun playing this back when I was a kid. Got a score of 258120. Made me pretty happy.
-The number of customers you can have in the spa actually has a capacity. By delaying collecting money, you can actually stall when the next customer comes in.
-A customer's patience does not decrease as long as you're holding onto him/her (you click the mouse on them and do not release it)
-Unintentionally, if a customer sits in a massage chair and waits for cream, their satisfaction goes UP rather than DOWN. Meaning the longer you leave them in a chair the better it is for their satisfaction. Obviously this does not work if you put them into the massage chair unwillingly, but it does make the last few rounds unintentionally easy. This does have a somewhat negative effect if it leads to the customer waiting for the massage chair wait extra long, and also on the final score as it takes your time in each level into account. So sometimes it's better to have a few angry customers.
Good for a few hours, but obviously things need a little polishing like the upgrades everyone's been talking about, limited storyline, all the customers behave the same except stats, why not have an appendix where you can inform players which customers are like which? I recommend taking a look at Cake Mania and see where they went with the directions. They had upgrades, different customer behaviours, and a storyline, which while not a masterpiece of shop upgrading, had a lot of clever/diverse implementations that shouldn't be hard to do in a game.