The Breden Preserve, 264 acres on Milford Bluffs, plus 30 acres preserved by the Nature Conservancy, provides a charming, and fairly easy walk to fabulous views of the river. (Driving time is 31-60 min, Rated Intermediate for elevation gain and because trails are poorly marked)
The preserve is great fun, with a fantastic loop hike at its core, which you can extend in various ways. There’s about 600 feet of cumulative elevation gain: some of the climbs are quite steep. There are also a couple of stream crossings. Usually you can just hop over stones to cross, but in high water, you may get your feet wet. (Driving time is about 75 minutes, though the drive, mostly on Rt. 29 is beautiful and stress free; Rated for Experienced hikers because of elevation gain and steepness)
The Sourlands is one of the few hiking spots close to Trenton where you can experience reasonable elevation gain, second only to the Ted Stiles Preserve at Baldpate Mountain. Yes, it is about 40 minutes away (compared to Baldpate’s 20 minutes) and in the Sourlands you gain up to about 350 vertical feet on the steepest trail (compared to Baldpate’s 425). But, the Sourlands have miles of reasonably challenging forest-hiking trails to enjoy… There are trails suitable for all levels of walkers from Beginner to Advanced. Beginners can do some short loop hikes near the parking lot, while Intermediate hikers have miles of trails in the southern section with moderate grades. (Driving time is 40 min; Rated Beginner to Experienced depending on where you go)
The Switchback Trail delivers 425 vertical feet by itself, and you can achieve a cumulative elevation gain of nearly 1,000 vertical feet (well, OK, 950) by combining it with other trails. Rated Experienced for elevation gain)
Just 45 minutes from Trenton, across the river in Bucks county, is a spectacularly-scenic ridge-hike along Tohickon Creek. This is a moderately strenuous walk… most of the climbing (a couple of hundred vertical feet) is in the beginning along a well-graded road. To truly enjoy the hike, explore the side trails along the valley. (Driving time about 50 minutes; Rated Intermediate to Experienced, depending on how adventurous you feel in exploring the side trails)
Utterly beautiful. 12 miles of hiking trails with up to a 400 foot elevation gain. 20 minutes from downtown Trenton. Need I say more? This is the jewel of the Mercer County Park system, and one of the nicest small parks in all of NJ. (Driving time is about 20 minutes; Rated beginner to Experienced depending on where you go).