AFNI's Graphical User Interface has a deep feature set.
You can slide between overlay and underlay. Enable this mode by pushing 3 or 4 with an image window focused.
For underlays with a time dimension (multiple values), the “graph view” can be used to visualize values at each time. Push v on the graph window to auto-scroll through time points (like a video)
Middle click on the p-value to specify a numeric value for thresholding. No need for the fine mouse motor skills to set the threshold slider perfect.