One thing you can do from Mew Lake is rent bikes at the Two Rivers Store and go for a ride along the Old Railway bike trail, making stops for a couple of short hikes. The western terminus of the bike trail is at the south end of the main loop of the Track and Tower Trail, so from there you can make a short hike to the lookout over Cache Lake. A short bike ride east of Mew Lake, the bike trail intersects with the LoTR–Provoking portage, which in turn intersects with the Highlands backpacking trail near the side trail to the Starling Lake lookout. So those are two scenic lookouts that you can use the bike trail to make short cuts to, which be long hikes if you were going on foot all the way from the Hwy 60 trailheads.
If you're just going to do the conventional thing and drive to hiking trails, I agree that Centennial Ridges is the best for scenery. If that's too long, the Lookout trail is a short dash of impressive ridgetop scenery. And consider the Mizzy Lake Trail in the early morning for wildlife.
If they're adventurous they could probably do a one-day canoe loop Cache-Head-Head Creek-Madawaska-Cache. Otherwise maybe just a short paddle on the Madawaska upstream of LoTR, or Costello Creek from the AO dock on Lake Opeongo. Even renting at the P Store, launching at Smoke and just paddling to Little Island Lake and back (circling the island?) I think would be a nice taste of portage country.
