We love Arnside. Provided you get the tide times right the walk along the shore with views across to Grange Over Sands and Heysham is fantastic. Once you reach Far Arnside the path up to the Knott gives stunning views over the Lakeland Fells. For a relatively short walk, I can't think of much better! As long as it's quiet much of the walk can be done with your dog off lead, but don't let the dog (or yourself) stray from the shoreline or you risk being caught in treacherous quicksand or stranded by the rapidly advancing tides.
A month or so ago we had what was probably our best trip there. The cold weather seemed to have put others off so we saw only a handful of others en-route. If you've never been, go!