After just returning from the Grand Princess over the Holidays, I will give you my 2 cents about the Lotus Spa.
Go on the Spa tours that are offered shortly after boarding the ship. They hand out coupons and entry forms for door prizes.
The drawing for the door prizes happens later on, and you must be present to win, so it is worth it to pop back up to the spa for the drawing.
The really push you to book on the sea days, they will almost act like there are not any appointments available on the port days.
Towards the last 3 days of the cruise you will see a lot of specials offered in your outside the cabin "mailbox". They include some great specials like a massage and pedicure, or a trio of different things to try out
I had a pedicure the very first day, since I ran out of time at home. It was ok, chipped fairly quick, but looked nice the bulk of the cruise. Really got the push on products at the end of the pedicure.
I had a facial that was wonderful. But, it was ruined by the pushy product talk.
The best thing I had was the hot stone massage. It is spendy, but worth it. I also had given my gal a heads up that I did not need the product push at the end. So that made it more relaxing.
I did get a coupon towards the end of the cruise for the hot stone massage, so that might be worth waiting for.
Stick to your guns about now wanting the product push, also be firm about when you want your appointments, do not let them push you into a time slot you do not want.
Enjoy, and treat yourself, you are worth it!