It's too late as a tip for this year, as it all ends tomorrow (June 24), but it's worth pointing out that 'Sant Joan' is also the date in Alicante for its smaller-scale version of Valencia's famous Fallas, with the elaborate papier maché effigies built by competing neighbourhood groups being burned.
Alicante itself is well worth visiting as a city (and so easy to get to from Northern Europe in particular as the gateway airport to the Costa Blanca resorts), although during this week it will be busier and pricier as a premium for taking in the festivities as well.