My wife loves the hydro spa but I can take it or leave it. I have used it after a massage but I would not pay what they charge for it. It is just one of those things that is up to your personal preference. I'd give it a try before buying a cruise long package.
General cruising tips:
Bring a nightlight for the bathroom.
We like to put a colorfull picture or poster on our door. It makes finding your room a bit easier.
If you drink, Hollandd America does allow you to bring wine onboard. A bottle is nice to have on the balcony during sail away. Box wine packs easily and my wife likes to have a glass while getting ready for dinner.
I like to bring an insulated travel mug. Usually I get up first thing and walk up to the buffet and bring a cup of coffee back to the room and sit on the balcony or go for a walk around deck. The travel mug holds more, keeps it hot longer and does not spill on the walk back to the cabin.
Leave some extra space in your suitcase or pack an empty duffel bag to bring home souvaniers.
Must have items: bathing suit, sunscreen, sunglasses, lightweight raincoat, light sweater/sweatshirt for sometimes cold theaters onboard.
If there is no coffee, orange juice or if they are out of mugs or glasses at the buffet walk around to the other stations. Sometimes one station is out of something while the other has it.