Miller State Park in Peterborough, NH is the oldest State Park in New Hampshire, protecting a portion of Pack Monadnock deeded to the state in 1891. The park is pet friendly and has some great easy and rewarding hikes, as well as an auto road to the summit.
There are two hiking trails leaving from the park entrance to get to the Pack Monadnock summit. The Marion Davis and Wapack trails are both around the same length. I found that the Wapack trail felt longer and had some rocky terrain, but also had some great views from the trail. Both trails are very easy hikes. Leashed pets are allowed in the park and I met lots of dogs and dog walkers on both trails. The Raymond Trail starts outside the park on East Mountain Road, but is also a good hike and takes about the same amount of time as the trails leaving from the park entrance.
At the top of the mountain there is a fire lookout tower with a great view and you can see the Boston skyline on a good day. It was pretty hazy when I was there, but the view was still great. There are also lots of shaded picnic benches at the top, and running water. There are some more views and scenery from the Summit Loop Trail.
The auto road is a quick and easy way to get to the top of the mountain, which makes the park even more attractive as an easy picnic spot. Do be careful in driving up because I did encounter lots of people inexplicably walking up the road.