Forgo toys, if possible. Without distractions, your tot and his friend will have more chances to interact. Go outside to play tag or use trees for games of peekaboo. Or turn on music indoors and have a dance contest.
Avoid potential conflicts. If you feel you can't go without toys, make sure you have plenty on hand. You don't need two of everything; just bring out toys of which you already have multiples, like building blocks and stacking cups. And take a pass on dates if the kid is a hitter or biter.
Watch the clock. Schedule playdates during hours you know your baby is at his best (like after naptime). Also, consider serving a snack beforehand to stave off the crankies. And while you may enjoy a two-hour marathon with your closest girlfriend, your baby will probably max out after one hour.