I have 3.5yo twin ds' and a 1.5yo dd.
-I take them everywhere with me, because everything can be made an interesting trip! Even just going to the grocery store, they are fascinated and we talk about colors and shapes and food.
-I exercise 3-4x/week, at the YMCA, where there is childcare, so I get to work out, and they get to meet other kids and play for an hour or two.
-I belong to a moms club and attend a lot of activities -- just as much for the kids as for me...