Lysekil is where I spent every summer growing up, and I really like the place. It is a typical Swedish summer town on the west coast with nicely curved cliffs and a few beaches.
The town itself is very different throughout the year. Going there during the off-season gives you the impression that it is very slow and rather empty, while the population grows by several hundred percent over the summer and everything comes to life. I like both experiences, one for its calm and almost meditative feeling when heading down to the ocean completely alone, and one for the joy of enjoying the summer after a long cold and dark winter.
Go here during the summer and enjoy Swedish summer with the Swedes when we are all happy and enjoying our vacation. Enjoy an ice cream by the ocean, take a dip, play mini-golf, and eat something made with the local seafood.