Police, courts and defense, with defense also including the 'most important' vaccinations for all who enter, whether through ports or by birth. Some reasonable upper bounds on the destructiveness of weaponry and toxins sold over the counter are likely also necessary, even for purchasers who haven't per se 'done anything wrong'.

There also need to be some regulation of how individuals use 'commons' like air, water, electromagnetic and sonic spectra etc., as well as land use rules, to avoid one person land locking others, parking their mile long yacht alongside Golden Gate Bridge, blocking bay entry, and such.

Some minimum levels of diplomacy, aimed at providing Americans, both individuals and organizations, 'fair and reasonable' treatment abroad would be nice, as well, especially if backed by military muscle.

I'm sure there is more, but those are the ones I can think of right now.