In the northwest of the island, on the coast, you will find Puerto de Sardina. This little fishing village is nice to visit. The beach has pretty golden sand and Sardina itself is picturesque.
There is a restaurant there (try the sardine recipes!) and you can also scuba dive. You can find some more information about diving in Sardina del Norte here: http://canary-islands.greatestdivesites.com/gran_canaria/sardina_del_norte
Galdar is not too far from Puerto de Sardina and a lot of people who visit one often visit the other. The north of Gran Canaria is very different from the resorts in the south. They are like different worlds almost. In the south you have the busy nightlife, the shopping centres and the packed beaches. The north of the island is more tranquil and quiet; you really ought to visit the north if you are on holiday in the south, just to see the differences.
Sardina is accessible by hire car or bus. If you want to get the bus, you will have to get to Galdar first and then get a bus from Galdar to Sardina. It is not a long journey by bus (and it is only a few euros) but, as you would imagine, it is much faster by car!
There is not a lot more to say about Puerto de Sardina. You can visit one morning or afternoon, for an hour or so. I do recommend the restaurant. It is pretty basic but the seafood is great.