I wouldn't put any water under there because of potential mould issues (in fact, there looks like there's a bit of mould already -- is there?). You could tile the bottom of it and put a strip of LED lights under the glass. What did you have in mind when you initially thought of the idea?

Flooring materials can be different and you cannot use the same cleaning material for all,and of course, it always depends on the use of them.Although,you can clean the tile,marble or any surfaces by yourself but its recommended to consult the expertise of floor cleaning service provider because they are professional in that services.Along with the regular swabbing and sweeping,one also undertakes additional functions to ensure optimal cleaning.This includes waxing, burnishing,stripping,buffing,and scrubbing.Our friendly staffs provide High-speed polishing that will give your floors a professional shine.

Instead of the solid grey, you can go for sketches in grey with flashes of mustard/ yellow. Or, look up these links for wallpaper ideas https://goo.gl/images/OrCtY1 https://goo.gl/images/0ZpAmU

I have some suggestions if you are thinking budget friendly changes. Make a photo gallery with different size frames and arrange them parallel to the stairs, so you can enjoy each photo as you climb up. And for the area that will be on a level with your upper floor, try some pretty decal that will connect the photo gallery to upper level. And last, if you have budget add some beautiful hanging lights in the stairwell. Also think about repositioning your refrigerator.