The gift tags are even easier. use cards with cute designs on the front. Cut off the front and discard the back. You can add a sentiment using rubber stamps to the front. Punch a hole in the top for string. Rubber stamp on the back To and From. using glue and glitter ...decorate the edges.....or you could ink the edges. If the card is made of slick paper use permanent ink. Add ribbon or string to the top and you are done. You could do a bunch of these ahead of time and have them ready for the holidays. Or you could box a selection of them to give as a gift.
Here are some more web sites with crafts using old cards. What have you made with them?
http://www.ehow.com/how_9171_recycle-reuse-holiday.html
http://www.makingthishome.com/2009/03/26/how-to-reuse-old-birthday-cards/
http://www.makingfriends.com/readers_cards.htm
http://www.tiphero.com/tips_3596_14-ways-to-reuse-old-greeting-cards.html
http://familycrafts.about.com/od/greetingcardcrafts/Greeting_Card_Crafts.htm
http://www.allfreecrafts.com/recycling-crafts/greeting-card-box.shtml
http://familycrafts.about.com/od/greetingcardcrafts/tp/GreetingCardCrafts.htm