Late May through June (or so)
Check out an aerial photo of the Delaware River at Trenton, and you see a lot of structure.
Spring through Fall, this structure becomes home to a nice population of smallmouth bass ready and eager to take your fly, lure, or bait. Fish up to a couple of pounds are pretty common, and larger is possible.
Best smallmouth fishing is probably May-September, excepting August and perhaps late July (which can get too hot for good fishing), depending on the year. Usually by May the water levels in the Delaware are low enough to enable wading of most of the structure accessible from the PA side. Rainfall upstream of Trenton can change that day-to-day.
Just walk across the Calhoun Street bridge (from which this picture was taken). There’s nice wadable structure up and downstream. If you drive, there’s parking just north of Calhoun Street off River Road. Your NJ license is valid anywhere on the Delaware.
When you cross the bridge, wear your polarizing sunglasses, and check out the large runs next to the bridge piers. You’ll often see large numbers of carp schooled up there. Some of these fish are HUGE, 25 lbs. and more.
Carp are spooky and picky…tough to catch on a fly or lure, easier on bait. But well worth going after, if you’re patient and cast well enough.