I used a gradual ND filter. 2 stacked in fact lol. The sky was simply not very good that day, and as such I had to boost the saturation more than I would usually. I have a lot of shots from this set that are long exposures (2-15 seconds or so) like you said, but they are just flat and boring unfortunately.
Damn skies.. not showing up to the party of my night off work.