Hello. I have been in Benidorm when Halloween has been on. It was a few years back now though, but I think most places hold some sort of Halloween 'party'. And a lot of people walk around the streets wearing Halloween 'attire' .... hats etc.
There is a nice Fancy Dress Shop in the New Town area that sell, or used to sell, good Halloween outfits and bits and bobs etc ...... assuming this shop is still there of course.