A big part of it is just you getting used to the different sound and feel of the bike at those speeds and freeway riding in general.
Those little 234 c.c. air-cooled Honda engines have been around for a long time and are virtually bullet proof. The Nighthawk *will* comfortably cruise at an indicated 65 m.p.h. all day without coming to any harm; I’ll agree, it does get a little scary at the top-end (~75-80) though Not an ideal bike for the freeway, but absolutely capable of it so long as you stay in the slow lanes. It is happier in town and on the country back-roads though.
Yes, the high revving at speed can be disconcerting at first, and having recently bought my first inline 4 (which red-lines at 12,500 r.p.m.), I can appreciate your concerns. At ~6000 r.p.m. my bike sounds (to me) like it’s revving really high, but it still has plenty of headroom if I want to take it there (which I haven’t yet). Quite a different experience from the much lower revving twin cylinder engines of my previous bikes. Like I said though, the Nighthawk 250 will happily cruise along at 60-65 all day long. Once you’ve done it a few times for, say an hour or two, you’ll settle down to it I’m sure. I had one as my first bike and am still sorry I had to sell it. Beautiful little bikes IMO.
Anyway, for second (or third or fourth, etc.) bikes I’d suggest looking at 500-650 c.c. standard or sport (not super sport) bikes e.g. Suzuki GS500F, SV650, GSX650F, DL650 (V-Strom) or Kawasaki Ninja 500, ER650, Versys or Yamaha FZ6. Other bigger, heavier, fast enough, but not scary fast bikes to consider would be the Triumph Bonneville and Harley-Davidson Sportster 883.
Cruisers? Maybe the Honda 750s, Suzuki C50, Kawasaki Vulcan 500 or 900, Yamaha V-Star 650 (maybe a little under-powered).
That’s just off the top of my head. If I’ve missed anyone’s favorite, I’m sure they’ll be happy to add to the list.
For many, any of the above would be the last “upgrade” they’ll ever need in terms of performance.
My favorite review site is:
http://www.powersportsnetwork.com/enthusiasts/review_list.asp?vehiclecategory=1