The most obvious large expenditure full of waste is the military. Any serious effort at reducing the federal budget deficit and finding ways to save money has to start with the military.
If we quit fighting wars and closed military bases in other countries, we would save billions of dollars a year.
Most of the other ideas about saving money are just nickles and dimes compared to military spending. If we're serious about saving money, we have to look at the military first, since over half of the discretionary federal budget is devoted to it. If we're not willing to touch the military, then we're just fiddling around.