Until age 2 or so, kids tend to play side by side rather than with one another, but there's no reason they can't have playdates. Some games that'll help encourage little ones to play together and have fun doing it:
Follow the leader. Using sidewalk chalk, draw oversize shapes and animals (whatever the kids are most familiar with or starting to learn) on your driveway; connect them with straight and/or curvy lines. The kids can simply walk the chalk paths and name the symbols as they reach them, or you can challenge them to hop to the blue square or waddle on over to the yellow ducky.
Search party. Hide several of two different types of objects around the yard -- such as a bunch of balls and a few dolls. Assign one object to each child, then send them all off to hunt. Give each kid a sample of the object she'll be looking for and a bag or basket to hold her discoveries. Be sure to "hide" things well within sight.
Foot fancy. Set up a wading pool and let the kids paint their feet with washable paints, then step in the pool, wash off, and start again. (Always supervise around water.) You can even let them paint the bottoms of their feet, step on paper to make their footprints, and then hop back in the pool. Later, they can try to find their dry footprints by matching them up with their feet.