name-that-baby-picture :-) - DH's extended family (MIL was 2nd of 11 kids) has a family picnic each year and one year one of his aunts got a few poster boards and put a bunch of baby/kid pics for the 3 generations represented at the picnic and everyone had to guess the identities of all the kids in the photos.
Another easy one to do for all generations is to fill up a jar with a pre-counted amount of something (jelly beans, crayons, pennies, etc.) and everyone tries to guess how many of the item are in the jar - closest guess gets a prize.
Bingo with white elephant prizes (every family brings a few) is also a regular at the family picnic
Pictionary in teams, using butcher paper or a large dry erase board for drawing on is also a big hit. An alternative is to have play-doh and people have to try to construct the word instead of draw it.