Go to the Park or Playground
Parks and playgrounds offer many opportunities to encourage and develop gross motor skills, especially for younger children. Toddlers can try to negotiate over a variety of surfaces, children can learn and practice their swinging skills, pre-schoolers and school-age kids can climb ladders and monkey bars for strengthening, and older children can play a game of basketball or tennis if the courts are available.
You'll be surprised that even very young children love to play and be active if you give them the opportunity. Recently my four year old and I went to "play tennis" and my two year old was so excited to run after the balls. We all got a great workout.
Go on a Nature Hike
Put on your sturdy sneakers or hiking boots and head out on a hike with your family. Many local state parks have hiking trials with maps available at the information centers. You can wear your infant or toddler in a baby carrier or backpack so they can come too. Have your kids find a "hiking stick" and put together a simple scavenger hunt to make it even more fun for them. Don't forget the sunscreen and insect repellent.
Go for a Bike Ride
Make the most of your sunny days and ride bikes as a family. Strap on your helmets and get outside to ride. Stay in your own neighborhood, or pack the bikes in the car, and go to a local park with a trail. If you have little ones, you can use a bike trailer or seat. If you have a long driveway, you can even use sidewalk chalk to make "roads" for your young children to negotiate through and around. Toddlers can try to keep up with their older siblings with their push-toys or tricycles.
If it is too hot outside or rainy, you can still stay active inside with these ideas.
Use couch cushions, pillows, and blankets to make an obstacle course for your kids. This is especially great for younger children as it works on balance, strength, coordination, and problem solving while providing an aerobic workout. Make it more fun by taking turns with your kids to set it up. Play "leap frog" and hop or jump from cushion to cushion. You'll be surprised how much you'll be out of breath too.