Routing somewhat depends on where in NH you're going to. For southernish NH, crossing back near Niagara Falls and more or less taking I-90 is probably the quickest (though not necessarily the most interesting) route, of course making the appropriate adjustments to avoid the mess that is the Boston area. For northernish NH, it may make more sense to continue through Canada around the northern side of Lake Ontario.
A few possible destinations along the way include the Henry Ford Museum/Greenfield Village outside of Detroit (now called just The Henry Ford, I believe), which could easily consume two days at least in itself; Niagara Falls if you haven't seen it already; various nice things around Rochester, such as the Strong Museum of Play, the George Eastman house, Seabreeze amusement park (a neat smallish family-run affair), etc.; and Letchworth State Park, an hour or so south of Rochester. I'm sure there are several others as well. The Henry Ford and Letchworth would be my top suggestions.