I wrote equidistant contour lines generating plugin for ImageJ/Fiji. It adds to overlay (so you can transfer it to RoiManager if you want) contour lines of images. Comparing to Contour Plotter it is a bit easier in my opinion, since there is no need to specify intensity levels.
Plus it can color code it with any LUT.
It helps to see intensity changes in wide dynamic range that do not fit well into image LUT (spots on top and bottom).