An assortment of different size rotary files and sanding cones/discs (not sure of the correct name) works for me. The trick with the rotary files is they MUST be spun backwards in a reversible drill. If you run them in their normal direction they will bite into the wood and tear and crack it. Running backwards it still cuts thru balsa or plywood with ease and won't tear or crack even balsa as the tool exits the backside of the hole. Sanding cones like you might use with your Dremel tool don't care what direction you run them. We are talking small diameter holes here, 1/8" to maybe 3/4". For larger holes, 1" and up, a hole saw assortment works using a backup wood block as previously advised.