Sounds like an oxymoron: Serenity-Terminator. Well, you get the idea. I spent a very nice hour (4:00 UT 4/30/01) working my way around the Western edge of Mare Serenitatis in my 12.5" f/5 Dob. I was working mostly at 200X and 400X in a 8 mm Radian without and with a 2X televue barlow but I had some very nice views (and surprisingly good resolution) at 120X with a Russell 13mm (still my favorite Russell eyepiece).
I started in the north with Rimae Hyginus and Ariadius (p34, Rukl). I enjoyed following these valleys along thier paths ultimately meeting Mare Tranquillitatis on its western rim. Hyginus is particularly interesting being formed in part from a number of little craters the most prominent of which is Hyginus. Hyginus was interesting to me because it appeared to have no uplifted walls despite the low lighting from the east. It was just a black hole in the surface with no brightening at the edge.
From there I moved to the northwestern corner of the Mare Serenitatis after a short look at the tortured region surrounding Hipparcus and Albategnius (p45, Rukl). In the northwest (Rukl, p13), I worked my way along the Montes Caucasus and spent some time exploring amung the craters Aristotoles, Eudoxus and Calippus. Egede, to the north and west of Eudoxus is interesting, I'm guessing its an older crater filled by a lava flow just short of the top of the rim which remains exposed. The Caucasus are also very tortured looking in the low light as well they should be. This is an extensive mountain range stretching more that 300 miles with peaks reaching greater than 18000 feet.
I was stimulated to take this trip by Vic Palmeri's message about the Rupes Altai. Unfortunately, I could not remember what structure he was discussing when I was at the eyepiece and never did look at it :^( Sorry about that, Vic, but thanks for providing the stimulus to look.