If you're looking for a meeting place or a good stop around ½ or halfway between Middlebury and Mount Sunapee, you can try Royalton, VT, which is almost an hour southeast of Middlebury, or about 1 hour northwest of Mount Sunapee.
Another possible place to stop near the midpoint is Sharon.
About ¼ or a fourth of the way, you can stop at Rochester, VT. This is about half an hour southeast of Middlebury.
About ⅓ or a third of the way, you can stop at Camp Brook, VT. This is about half an hour southeast of Middlebury.
About ⅔ or two thirds of the way, you can stop at Lebanon, NH. This is a little over 1 hour southeast of Middlebury, or about half an hour northwest of Mount Sunapee.
About ¾ or three fourths of the way, you can stop at Olympic Mobile Home Park, NH. This is about 1½ hours southeast of Middlebury, or about half an hour north of Mount Sunapee.
If you drive straight through without stopping at all, the total distance is 94 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 1 hour, 54 minutes.