I think the easiest setup is to use a copy stand with a digital camera. I've been working on a huge set of my wife's family photos, and this was the easiest and fastest way to get digital files of reasonable quality.
In my case, since I don't own a copy stand, I set up a small tripod on my large desk, with the center column reversed and the camera aimed straight down about 2-3 feet from the desktop. Using a 17-55/2.8 zoom, I can easily fill the frame with a 5x7, and get enough of the smaller prints.
Lighting is critical here. I use two speedlights, aimed at roughly 45 degree angles, so any reflections from the surface of the print won't end up in the final image. This requires a fair amount of tweaking, but it works. For larger prints I might have to use 4 lights, but so far I have avoided that. (You could use two gooseneck lamps with standard light bulbs, too, if you like. Gets hot, though.)
For simplicity and speed, I am very careful with my lighting, white balance, and exposure -- and I shoot JPEG files in camera, tethered to my iMac. (These are the only JPEGs I've shot in many years. Please don't take away my photographer card.)
Getting the photos to lay flat is a challenge. Be careful about using a piece of glass -- you can get a reflection of your camera lens if you aren't careful. (I had to redo a bunch of photos in the first batch.) Another challenge is photos with a strong texture, but I just let the texture be part of the final image.
The upside to setting up all this stuff is the speed at which one can "scan" hundreds or thousands of photos.