Major examples are the decentralized programs like welfare, employment services and environmental protection. Grant program are a major factor in the federal system. The grant authority is based on exchange, not on the Constitution. If a state does not like the terms, it should refuse the money. Interstate relations within the United States are handled much like foreign affairs.
States can negotiate and enter into treaties with other states.
A historically important example is the treaty negotiated in 1785 between Maryland and Virginia on use of the Chesapeake Bay. Sometimes the states negotiate under the provisions of a national law. The interstate compact on low level radioactive waste that led to the tentative selection of Riga Township for a possible site for disposal was in this category. Because Michigan failed to find a site, it was expelled and the responsibility now belongs to Ohio. Another example of an interstate treaty is the Port Authority of New York and New Jersey.