Storage in Spain?

Every year we fly to Malaga 2 or 3 times and stay in different resorts. We seem to be bringing the same stuff with us on every flight . With 2 kids, our suit cases are full of swimming inflatables, beach toys, sun cream, hats, towels, etc etc

Is there any place that we could store stuff at a low cost in or near Malaga Airport or even further a field?

You will have no difficulty picking up storage space but of course you will have to pay. Why shoot yourself in the foot? The stuff is not expensive (especially in Spain). Leave it behind or dump it. We do this all the time to avoid paying hefty baggage airline fees and free up space for some drink.

We rent out our place in Mojacar Playa (Costa Almeria) and people tend to leave behind those styrofoam boards, beach shovels and beach buckets, snorkels, inflatables = the lot. You are welcome to them.