I like the gantry crane a lot (magnetic, not claw type). It is more than push and watch, it lets kids move the boom, run the line down / up, pick up and drop the "load". 2 of my 5 year old grandkids figured out how to use it in less than 10 minutes and they love it, usually spending probably 15 - 20 minutes on it.
Another is the sawmill - it can easily be set up in advance, starts off a pushbutton, runs for several minutes, makes some neat sounds, and, amazingly, "turns logs into boards". 5 year olds and 55 year olds really like it.
Another one is the track repair section (that you could put on a separately powered siding) where there are workers moving as they move rails and weld track.
I also like considering some of the Miller Engineering signs that are flashy and build up to different levels with a product that a 5 year old might recognize or enjoy: Life Savers and Cracker Jacks are 2 in particular that could be run with and on / off switch, they are colorful, fun to watch.
I am glad that your are enjoying your dad's trains with your son - it's quality time that will create memories forever for both of you.
Go for it.