Having small kids means having to keep track of lots of small toys, both at home and when out and about. Instructables user lizzyastro created a combination play mat and toy bag so that your child has a convenient place to play with building blocks and when finished playing pull the draw cord to turn the mat into a bag.
You'll need two different pieces of fabric: the outside surface should be a weather-resistant synthetic fabric and inside can be whatever your kids would like. Cut out each fabric into a circle, sew them together, add grommets and finally run a drawstring through the grommets. If you're already setup with sewing gear you'll just need 1.4m of each fabric and a grommet tool.
If you like the idea but don't have sewing skills you can also buy a commercial play mat/ toy bag for $US45.
Toy Bag and Play Mat in One [Instructables]