Not sure if they would be considered unconventional uses or not, but here are a few more:
Packed with fill used as a stepped retaining wall
Empty, various sizes, staked down as a tangle foot
Stacked empty, securely tied together as a rebound stopper wall
Filled with concrete, with an eyehook embedded for use as an anchor for several applications:
(Off shore boat/raft mooring, floating bridge anchor, moveable shore tie off point, portable structure anchors, stock animal tie out anchor (smaller ones as drags), dog cable run trolley anchors, road drag, support anchors for unstable items, etc.)
Stacked high, empty, not too stable, as obstacle that will fall on those trying to pass by
Lined up side by side, on the tread, with a cable run through them as another type of rebound stopper wall
Filled with rock, debris, broken concrete, etc., and rolled down a slope in mass as an attack charge disrupter
Small to mid sized tires filled with moderate weight, fuel in ziplock bags, various other products, as siege engine ammunition
Weights to hold down/protect shingle/tile roofs
Weights to hold down sheet plastic/tarps over garden spots and other ground areas needing protection
Weights to hold down/secure tarps over hay stacks, etc.
Just some off the top of my head.