I wouldn't bother with the bike, any tracks around Harrington will be sand and not especially fun on a bike.
Take the fishing kayak. You can have some luck in behind the breakwall or head out past the boat ramp and into the main river.
The ocean kayak could be an option, you could duck across to Manning Point for an icecream.
I was reading that a large great white was spotted off Crowdy a week or so back and a rather large tiger shark was landed at Foster, so don't become lunch for something further up the food chain.