Very high redshift galaxies are readily detectable with NIRCam:

NIRcam's coronagraphic mode will enable studies of circumstellar material and planets. Plot from Green et al. 2005.

Pop III SN (if they exist )  will be easy to detect. SN with M=175Msun will be 25 times fainter than the 250Msun SN shown below, and would require more time to detect but would still be easy targets. However, we do not know if the rate of PopIII SN will be high enough for JWST/NIRCam to find them.
Jupiter -size objects are readily detectable with NIRCam. The spectra plotted here are from Burrows et al.

Near-Infrared Spectra of KBOs with NIRCam Medium Filter Bandpasses Overlaid (see NIRCam Features for complete filter list).

The NIRCam filter set (see NIRCam Features) can measure brown dwarf and planet temperatures accurately. The models used are from Burrows et al.
KBO SEDs can be measured with JWST - the diamonds show 10-sigma detections using NIRCam and MIRI for 10,000 per filter. The hypothetical KBO is at a distance of 45AU, has a diameter of 200 km, and an albedo of 0.10.