When in Rome... do as Icelanders do. Expect wind, rain or clouds and rejoice when it turns out to be a bright and sunny day. Don't be surprised to see young men bravely wear t-shirts even when it's 12 C outside and sunny.

Also, if you want to save money, stay at self-service apartments so you can prepare most meals (breakfast or lunch) at home. There are a number of 24-hour grocery markets where you can buy food, etc. If you're a little adventurous, you can find whale meat in restaurants (and grocery stores). If you'd rather prefer to eat it in a restaurant, I'd recommend 3 (Thrir) Frakkar.

BTW, forget Blue Lagoon. Locals don't go there; instead go to one of the thermally heated municipal pools. There are several to choose from in the Reykjavik area.

Stockholm temps in mid-October can swing from 8 to 16 degrees.