There really aren't any chocies - route 9 to cross the Connectocut River, and over to Keene, then route 101 to Peterborough. If you have time, from Marlborough, NH, you could cut down route 124 to Jaffrey, then back up route 202 to Peterborough, this would take you on the south side of Mount Monadnock, there are some occasional nice views of the mountain from that route - between the trees.
Rte 9 to 101 is about an hour to Peterborough. It is a lovely drive. Drive up Main Street in Keene, go around the second traffic circle and then back down to 101 to see this attractive small city. Spend some time there, if you like. Nice shops and some excellent dining. As you continue on rte 101 there are beautiful views of Mount Monadnock as you drive around Dublin Lake. The route suggested by ffbrgst will add about 10 to 20 minutes to the drive. I could suggest intentionally twisted out of the way routes to add one or more hours to the drive, but it really is not necessary. The straight shot on 9 and 101 is quite scenic.
I'd suggest just getting there by using the quickest most direct route.
I don't agree it's quite scenic but for a few brief views of Mt. Monadnock. But that's subjective and the main point is that I also don't know a significantly more scenic way without getting really carried away and taking all day.
Especially after a day of driving on Rt 100 in VT.....no matter what you do it's not going to compare with that so I'd just get the last hour over with by going most direct.