For a pleasant but strenuous half-day hike to Mt Monadnock's summit, consider the 3.9-mile White Dot and White Cross loop. Just know that you won't be alone most warm weekends – it's a popular route. The shorter White Dot trail is typically used on the ascent, with your return on the slightly longer White Cross Trail. Both are rocky and steep.
Your reward on the boulder-capped summit is a magnificent 360-degree view of the region from the highest point around.