In the Target dollar aisle (aka my nirvana), I found some calendars with spots for a 4x6 photo on each month. I saw "My family," and "My grandchildren," ones. We grabbed two of the grandchildren ones for both sets of grandparents. I plan to order my free prints and put them in the calendars.
It's an easy, quick and super cheap gift. When the year is over, the grandparents still have 12 prints too.