If you're interested in neat topography, The Hopper is pretty awesome (I've only taken it up to Stony Ledge, but it was neat). A loop could be made, heading back down via Prospect.
If you take the Thunderbolt, there's a lot of neat ski history on and around it. The Thunderbolt was (and is starting to become again) a pretty popular alpine ski trail before WWII. At the bottom, you're also at the base area of a defunct, never completed alpine ski area - there's a path that goes flat from the Thunderbolt base across the base area of the ski area, passing a few never completed ski lifts, which feature lift towers that are now *lower* than the trees. You can also check out the summit of the area, which is close to the lower part of the Bellows Pipe Trail if I recall. If you look at that side of Greylock at just the right time of day, the sun reflects off the lights, which are scattered throughout the woods there.