Having picnic on the beach or the park which is free. Just bring your own food and drinks. I am sure the girls will enjoy a swim on the beach. Playing tennis, volley ball or hide and seek in the park with the added bonus of fresh air. Walks in the forest or beach. Hope these helps.
I like all of liesl's ideas. Also, if you have one of those dollar stores nearby (the ones where everything is actually a dollar) or a FiveBelow (everything five dollars or less), you can probably find little badminton sets, jumpropes, squirtguns, and other outdoor game stuff. You buy some of that stuff and you can have a blast without even leaving your yard.
$20 on floats then $0-$4 to get into a state park with swimming area. I have been to 3 lakes in a little over a month. Every weekend we go to the lake and swim. Often there is a nice sandy beach area. We love it and it's a perfect fit for our 10, 8, and 3 year olds. Also I second Dollar stores. Sand toys, water noodles. bubbles, water guns, floats, and tons of other cool fun in the sun items are cheap at the dollar store.
A bucket of water and a paintbrush is always fun on concrete. It's self cleaning and creative. Not as messy as the chalk for sure. If the girls do something really special, get that digital camera out and save it forever before it dries.
Also, show them how to make sun tea outside. They will love it once they ice and sugur it up more than the powdered stuff.