How could I incorporate sand with the beach theme I'm planning for my bedroom?

Generally sand is something I'd suggest you clean from your floor as opposed to using it for decorating, but if you want to put it in a vase to admire and remind you of days spent on the surf, I'd take three large vases in varying sizes and styles. Buy a few square vases and a cylinder. Layer each with sand and shells or fill up with sand and add a candle to the center. Keep in mind that there is different types of sand and they will look different in vases.

If you are having a beach theme in your room, there are a ton of other great ideas for stuff you can add. Add a large surfboard to your floor by choosing this area rug for a 'Surf's Up' kind of feel or this one if you feel like Riding the Wave right on your floor.

You can hang signs from the beach such as "No surfing" or "Lifeguard on duty" as decor for your room as well.

A beach room is a fun room, so use your imagination!

I love offices.  They make one feel so organized when at home. You can relax and get things done in an office that is properly organized.

If you can't paint to add color, you need to add it in other ways.

Given that you have a desk you'd like in there, as well as needing a place for your cats, I'd suggest making it into a mini-office/rec room.

Place your desk and chair in the middle of the room, with the back to a a window. Be sure your desk is facing toward the door. When your desk is facing the wall, it gives a very negative view of the room. Depending on your budget, add 3 large book shelves behind your desk. You can buy different style of shelves from Ikea, and they are very economical as well as stylish.

Hang some curtains on your window. This increases the cozy factor. With a black desk (and plain walls), I'd focus on colors such as taupe, dark brown, or light cream. With no color on the walls, you can get really bold curtains that will add color to your room. With dark browns and creams, you could have a neutral curtain with orange or red accents.

On your floor, add a large area rug. There is nothing that says 'Welcome home' more than an area rug. Try a large rug such as this neutral with burnt orange/red accent such as this area rug. It will look great under your black desk.

Add a cushion to your desk chair. Even if you don't sit with it there, it looks cozy when you walk into the room.

If you want to add a bit of extra to your office, put a flat screen TV on the wall facing your desk.  Hang several picture frames around the TV, creating an almost collage of frames.  When your TV isn't in use, you can put a screen saver on it that will only add to the ambiance of the room.

Hide your cat litter boxes by removing the closet doors and hanging a curtain across the expanse of the closet.  Your cats can come and go, and the litter boxes will be hidden from view.