When people hear I have been to the gym or worked out whilst on holiday they usually think I'm either crazy or obsessed. This is usually the attitude of someone that does not view exercise with the right mind set and has never regular trained as a lifestyle habit and only as a means to an end (or more commonly as a means to ease their guilt without ever really reaching any end).
If you train regularly as part of an ingrained routine (whether this is weight training, running, swimming etc) you will know that exercise/working out is something that gives you a mental release, makes your day feel productive from the onset, provides a sense of achievement, promotes ambition, teaches persistence, perseverance and that rewards can be attained from hard work and dedication. These are all factors that promote a healthy mental state and positively affect your mood in the moment and attitudes in many areas of your life.
Yes, there are times when you have to drag yourself up and get yourself down to the gym/track/pitch but as anyone who has truly integrated exercise into their life you rarely regret getting up and getting moving.
So if you exercise is a something that you can enjoy and gives you a feeling of satisfaction and achievement then why wouldn't you do it on holiday when you have much more free time than any other time of year.
Learn to live it and you'll learn to love it. All the best from Reykjavik!