If you are coming from Keene, take 101 east then 124 (either south or east) towards Jaffery. You will come to the Toll Rd trailhead first. There are many great loop options from this area. Going over Monte Rosa or Bald Rock are good optons. Also this lot is parking area is smaller then the main lot so less of a crowd especially if you try to stay off White Arrow.
If you cont. on 124 for about 4 miles the rd heading to the main parking area (headquarters) will be on the left. There is a sign. This time of year the weakends are very busy. The above hiking loops using Cascade and Spellmen are good. I prefer Cascade all the way to Pumpelly. Another good one is to use Parker, Lost Farm and Cliffwalk then cont to the summit.
If time is very limitted and you can only get out for an hour or two I would suggest skipping Monadnock and hiking Gap Mt. This IMO has the best views of the Monadnock area, mainly because you have an aweome view of Monadnock 4 miles away.
To get there take rt. 12 south from Keene, a few miles south of Troy, NH you will pass a dinner on the left, take the next left. (there is a sign for upper gap mt rd) The trail is just shy of 3 miles round trip.
brianW