Was about to post the same. Other weekend we went to Rother Valley. That was good, £3.50 to park car. Decent walk around the main lake, couple of parks for the kids. Cheap water activities for the kids (pedalo, boats, zorb balls etc). Last weekend went to Cusworth Hall for a few hours.
The waterpark part at Clifton is closed for the moment, so be aware of that if you are wanting somewhere for kids to splash about
Rivelin Valley - they have a nice little pool and water play area and the river is shallow enough for kids of most ages. Please note if you are wanting a water play area then forget Clifton Park - the company that ran it have gone bust - the new company that have took over had to order parts from Germany to get it up and running again. It will probably be out of action all summer.
Wentworth castle, or if your after a meal and drink , park up on sweet street in Leeds and have a 2 min walk up past Bridgewater and have a meal and few drinks on granary wharf. A few quality placed round there or Clarence dock.
Millhouses park pleasant for all ages. Closer to Barnsley there is Wentworth Castle, Wentworth Garden Centre, Cawthorne Park of course.
Langsett res, park in the car park at the end of the A616 before it joins the roundabout for the A628. Walk straight down the path across from the car park and through the wood (listed as Brook House lane on google maps) follow the path until you reach the river (The Porter or Little Don river on Google maps) if you turn right and follow the river for a few hundred yard there's a large open area on the river banks, not many folk know about it and its a great picnic area. http://www.flickr.com/photos/treeblog/3570494481/
We used to laike int dike darn Low Valley, adjacent to where the new school playing fields are now - hours of fun.
Caphouse Colliery Mining Museum. Free lots to see on the surface and can go for a cool down, underground.
I very much doubt it. I think its existence is only known to those in the upper middle class, earn in excess of 50,000 per annum and have trouble using apostrophes.