calling the interior of these four walls a master bath is a bit of a stretch, but it is the only bath in the house with a shower, and it is upstairs ... so i guess that means it counts.
we didn't do much, it's such a tiny space you can't really, but we did paint, yay white! and added a few shelves (ikea ribba shelves UPSIDE DOWN, best idea ever), a snazzy shower curtain (west elm, though not on their site) and a not lame towel rack (so important). also we replaced our hot water heater. this was by far the biggest upgrade of the entire house - quality of life wise. we had a total of three minutes of hot water when we first moved in. then sometime in february we decided it was too cold to not have a hot shower. new water heater = best investment EVER. we can now sit in the shower forever. i frequently come out pink skinned. it is wonderful.
maybe one day kind of soon we'll replace the sink and the pathetic piece of carpentry it's sitting on. maybe one day not so soon we'll add an actual master bath. until then i'm basking in the white walled brightness and it's all pretty ok.
ps - sea salt soap, it's the best ever. both mas and i are deeply, madly in love. i'd highly recommend it.