I have a concrete driveway with considerable weed growth in the
expansion joints.  When I pull the weeds out I get an empty gap of
about a half inch and it looks rather deep.  Can I fill these gaps
with play sand or will that prevent the concrete from expanding?

I don't think sand would prevent concrete expanding, but it might
absorb water and expand, causing cracking, if subjected to
freezing/frost. I'd suggest a specialist concrete joint sealant from a
builder's merchant. There is more than you would ever want to know
about concrete paving joints at:
http://www.pavingexpert.com/concjnt1.htm

Scroll down to "materials for concrete joints"

Owain