These are so simple, and because they're small you can even hand-sew them and it won't take very long. I made a tutorial here if you don't feel quite up to freehanding it. ☺
I have made some fancy things like jester's caps and knight's tunics, but even a beginning sewer can make good dress-up clothes.
- A long scarf or sash requires no sewing at all, and can be looped around like a toga, worn over the head as a veil, or tied like a belt!
- A cape can be made with a big rectangle and a ribbon across the top.
- Skirts are two rectangles with elastic at the top.
- A long robe/dress/tunic can be made with these simple instructions. This other site has more elaborate directions for essentially the same thing (including directions for doing a neck facing). (Please note that both sites are in adult sizes! You can make a child's tunic with less than a yard of fabric, and a full-length dress with under 2 yards.)
- Some hats are simple and some are more complex, but a no-sew option is to get (or make) a circlet the size of your child's head, then wrap it with lots of ribbons (wrap in or glue on some silk flowers if you like). Tie the ribbons on so they stay, but leave long tails hanging down...your little princess/fairy/gypsy/queen is bound to love it. A mob cap (second one down) is just a big circle with some elastic on it (or here's a fancier version). A chef's hat is a big circle gathered into a wide band. If you're up for a slightly fancier hat, try a bonnet.
Sock puppets are a classic option, as are finger puppets. Another option is to get some small stuffed animals from a thrift store, make a little hole near the tail, and stick a dowel (or pencil) up into them. Voila, stick puppet! Alternatively, get larger stuffed animals from the thrift store, make a larger slit by the tail, and pull out the stuffing for a hand-in puppet. Finally, try cutting a large gingerbread man-shaped piece of nice stiff felt, and then decorate it. Cut a matching back piece and sew the edges from one outer ankle to the other (don't sew up between the legs of course or you won't be able to put your hand inside!
Lastly, FlyLady has a list of "clutter-free" gift ideas (experiences, many of them free/homemade). Thanks to RasJane for the link!