Regarding the question of where to go, if I have an hour or two to spare whilst in the north London area, I take myself to South Kenton station on the Watford DC / Bakerloo line.
As already mentioned, it depends what sort of rail traffic the OP is particularly interested in, but South Kenton scores well for variety.
From the island platform there you'll see a wide range of different trains in a constant procession. From Anglo-Scottish trains, Avanti Pendolinos and Voyagers at the exotic end, to regular freights and LNR EMUs on the slow lines, to LO Class 710s and Bakerloo tube trains at the humble, work-a-day end of the spectrum.
Regarding other passengers South Kenton always seems quiet enough, but not deserted, in the off-peak and has never felt dodgy or threatening when I've been there. The only downside, if you're into recording train numbers, is you will sometimes get "bowled" (unable to catch the set number) if a Bakerloo or LO train has stopped on the southbound DC platform when something interesting goes past on the main lines.