My DC are 4 and nearly 2. We spend a lot of time doing normal things (walks to the park, jigsaws, drawing, puddle jumping, building ramps for Hot Wheels, etc.), but there is a lot of time to fill.
I thought it may be helpful to have a thread with a bunch of more "off the wall" things that parents do to keep the children entertained...
I stick on classical music (Flight of the Bumblebee, In the Hall of the Mountain King, Yakety Sax, etc.) and have the kids dance around the living room as strangely as they can.
Any other activities to try?