The easiest way to do a "keyhole" scarf is to knit 4-6" in garter stitch (knitting every row) then knit across half of the stitches and start another ball of yarn there. Knit back and forth across the rows- one side of the slit is worked using one ball and the other using the other- until you slit is about two thirds of the width of the scarf you are knitting. Knit all the way across the row then and snip off the second ball. Finish knitting the length of the scarf like you usually do and then weave all of the tails in.
is a free pattern at Lion Brand yarn that is an example.
If you can find some "fun" yarns that would suit each person, this is a fast way to have splashy gifts on a pretty small budget. Have fun!