Requirement 2a: "Visit a place that is listed as a National Historic Landmark or that is on the National Register of Historic Places. Tell your counselor what you learned about the landmark or site and what you found interesting about it."
I worked in cultural resources management before I became a teacher. I worked with properties listed in the National Register of Historic Places almost every day.
I'm suprised that the merit badge pamphlet for Citizenship in the Nation is so vague regarding the National Register of Historic Places (NRHP) and the National Historic Landmarks (NHL) [see the merit badge pamphlet pp. 43-44; also Resources on page 47]. It's hard enough for adults to know about this, let alone middle-school and high-school kids.
The NRHP and NHL programs are administered by the National Park Service (NPS). NHLs are automatically listed in the NRHP.
The NPS website is difficult to navigate to find this information.
Since I "may share (my) knowledge or experience" [merit badge blue card], here are some tips:
To search for a site on the NRHP look at NPS Focus:http://nrhp.focus.nps.gov
To search for a site that is an NHL:http://tps.cr.nps.gov/nhl/default.cfm
A vast majority of the sites listed in the NRHP are private property (i.e. somebody's house). Scouts should be warned about trespassing and respecting people's property and privacy - in other words, these types of places may not be worth visiting.
However, many NRHP sites are open to the public, including parks, cemeteries, post offices, courthouses, city halls, museums, and railroad stations. Most state capitols are also listed on the NRHP (a couple are also NHLs).
The forms for these places are free and available to the public. One can send an email request to nr_reference(at)nps.gov with the name of the property (official name the NPS uses [see NPS Focus]), and its county and state. Include also the requestor's name and address. The forms should be mailed within two weeks. Note that the forms for NHLs have been scanned and are available as PDFs from the NPS Focus site. Also, several NRHP forms have also been scanned as PDFs and are on the NPS Focus site depending on the state. There are around 80,000 places listed in the NRHP.
As for Requirement 2c: "Tour a federal facility. Explain to a counselor what you saw there and what you learned about its function in the local community and how it serves this nation."
As mentioned above, many federal facilities are listed in the NRHP and that may also be NHLs, including post offices and federal courthouses.
I hope this helps and gives you all some ideas.