The first and easiest way is to simply replace your existing door and drawer fronts without doing anything at all to the boxes. You don't even have to have an exact match. Take a close look at your cabinets now. You'll see that you have areas that are both lighter and darker than each other. A light to medium oak door will work just fine in almost every case.  A slight contrast can add interest.

Invoke a warm, modern look by installing the very popular almond or white euro style door with continuous oak rail pulls and hidden hinges on your existing boxes.  (Example: Almond euro doors on your existing oak or maple cabinets. Almond foreground, oak background.)

Give your kitchen a bright, cheerful, contemporary look. Remove the doors and drawers fronts and paint the boxes white inside and out.  White doors and drawer fronts with beautiful new brass handles and hinges completes the look! This is a very popular method. This kitchen can look either contemporary or country.

The fourth way is to veneer your old boxes with your choice of self-adhesive wood grain or color to match or contrast your new doors. This method costs a little more and takes longer because it includes laminating or veneering the boxes, but the results are well worth the effort.