What I have done on some past cruises is take my own mud masks in a tube or plastic jar, hair treatments and then shower and apply. This is done ONLY if they cruise ship allows you to go into unlimited areas of their spa. For instance there was no charge for the Dry and steam sauna on the Celebrity Millenium when I was on it a few years back. I also was able to use the large cool salt water jacuzzi inside the atrium. This may have changed by now but I look for this each time I cruise. During some part of my vacation I take out time (ususally on a sea day). To go to the women's side of the spa.. with my swimsuit on. Depending on if I want to fully shower there (the shower stalls are always a LOT more spacious than in your room.) I have put on mud mask and also creame rinse or hair mud packs. Sat in the hot sauna for a bit then gone right to the shower. I usually take my own shampoo, razor and whatever else I need to at least get me clean afterwards. Then I return to the cabin and dry my hair and dress. I usually take my own more powerful hair dryer with me on each cruise. I have found the wall mounted units to not be strong enought for thicker hair and also the handle on the unit gets EXTREMELY hot and burns you. You have to end up putting it back in the cradle and letting the unit cool down and then drying again. This takes way more time than I want.
Other tips.. they now have Lays potatoe chip cans that are made of plastic.. kind of like the stackable pringles.. only this is plastic. I take the label off the can and wash and dry it. You can re-cycle and re-use things like this to pack jewelry or take a portable snacks on shore or while travelling. The plastic lid just snaps on and off.
Yet another tip.. use old underwear that is about to see the trash can. Use this on your days you are going to be out on a HOT shore excursion.. after using .. or at end of the trip.. simply toss it in the garbage and you don't have to carry it back home. Same goes for other articles of clothing you don't mind leaving behind.
Just my ideas.