Cowboy
A cowboy or cowgirl fancy dress costume is a great idea for dressing up in on a rainy day. The child should wear jeans, boots and a shirt, preferably plaid or checkered. Tie a bandanna or kerchief around his neck. If you don't have one, you can improvise by using a clean napkin or dishcloth. If the child already has a cowboy hat, she can wear this with the costume, but it's not essential.
Ghost
All you need to make a child's ghost costume is an old, white sheet, or one that you're willing to sacrifice for fancy dress. Cut holes for eyes and a slit where the child's mouth is so he can breathe properly. If the sheet is too long, cut it in a zigzag pattern along the edges so that it's an appropriate length. Ghosts don't just have to come out on Halloween; you can make this costume any time of year.
Princess
Every princess needs a beautiful dress, so put your child in her prettiest one. You could put an adult's dress on her, if she doesn't have anything suitable. It would be oversized, but that doesn't really matter for just playing inside. Put a crown or tiara on the little princess. If you have a store-bought crown, use it, otherwise make one out of cardboard and then cover it in baking foil.
Cat
A cat fancy dress costume is another idea for dressing up on a rainy day. Dress the child in black leggings and a black T-shirt. Cut black triangles out of construction paper or felt to make ears, and attach them to a head band. Draw on whiskers with face paint or black eyeliner. Alternatively, you could make a cat mask with a paper plate as the base. Attach one leg of a pair of woolly tights as a tail, using a safety pin.