The main town of Tenerife, not so tourist friendly, so your day needs planning. Free shuttle bus to the port entrance and an easy walk to the town centre from there. A trip to Mount Teide (through toursbylocals.com) is worth arranging.
There is cheap hire car available dockside in Arrecife and a full days hire through Avis, Hertz, etc., with petrol and CDW was only 45 euros. Made the most of this and drove around all of the south of the island stopping at Stratvs Bodega for a tour and wine tasting. The wine growing process on this volcanic island is different, as are the wines, and this was a day well spent. Arrecife is small and not particularly exciting!
MALAGA (not Seville). Malaga was excellent, lots to see and do in this manageable city that is walkable and interesting. Though we were there on a Sunday all the museums etc., are open - though not the shops. We planned our day taking the advice of Tripadvisor and going to the No 1 attraction in Malaga: the Museum of Glass and Crystal - a private collection of more than 300 pieces of glass arranged with furniture, pictures and decorative objects in rooms corresponding to different historical periods. The 18C house in which it is set is typically Spanish and has been lovingly restored. Our charming, erudite English guide/owner gave generously of his time to make this the highlight of our trip. The museum is a ten minute walk for the shuttle bus drop off point up past the cathedral. After this excellent morning we had a really great lunch in nearby Restuarante Vino Mio. A day to remember.