Fishing , you can buy telescopic rods that are cheap enough to be used on holiday, you can buy barbless hooks and past experience says definitely do this unless you can cadge a disgorger off another fisherman ! You probably won't catch anything, the fish have a knack of nicking the bait off barbless hooks, but its fun trying.
Crabbing, any harbours near by ?, we buy cheap tins of mackeral or streaky bacon for this, and get the decent drop nets and don't forget to hold on when you chuck it in
. Crabbing takes up at least an hour, there will be other families doing the same, usually what happens is the parents take over and the kids play amongst themselves.
Rockpooling , this can take up hours, bucket and net is all that is required, there is always that one pool with the 'massive' super fast fish that you have to join forces with a dog walker and another family to catch.
Rock balancing, see who can build the biggest stack, DS is too cool for this until I start to get a decent sized one going and then he can't help but join in.
Looking for heart shaped stones on the beach, its suprising how many there are 
Looking for sea glass, this can take a while too if you are quite specific about the quality 