This is a great little article. It suggests a handled medicine ball, but you can you a dumbbell.
http://www.acefitness.org/blog/5721/6-standing-ab-exercises?utm_source=SilverpopMailing&utm_medium=email&utm_campaign=Health-eTips-11-19-2015&utm_content=Health+eTips&spMailingID=24030239&spUserID=NjU5NTYyNDUwNDcS1&spJobID=681840104&spReportId=NjgxODQwMTA0S0