The Duinkerken=Dunkerque citybus is also free and connects even into BElgium and that famous coastal tramway there.
LOcal interurban BUSes with a for FRance even fairly extensive network cost only 1€/ride. Google for ´autocars arc-en-ciel - Hauts de FRance´
Since they merged the former smaller regions into those larger ones- used to be North/Nord de FRance- now ´Heights= hauts de France-also with former Picardie this has also affected the TER local rail networks. Though it seems this region has extensive low-fare offers for certain groups(even for me as a senior from out of FRance) these are-or seem to be nowadays mostly train-specific-also quite a few 1 0r 2€ fares for shorter trips at weak times-but that all need to be preorganised and with limited nrs for sale
The OLD North used to offer a kind of cheap daypasses at certain occasions, like EURo mobility week or open monuments day or the like-very often in September weekends. No idea if still on-but check the sncf/TER website for this region and can subscribe to have any special offers mailed to you.
PS-if asking for this like for FRance etc its always worth to mention if you qualify for such special groups, like senior, junior/student, big family etc. Just because of all those specific offers