Holiday destinations, including colder climates, are usually a lot sunnier than those to which we are accustomed. It has been well reported that exposure to the sun can damage your skin, making you look older and increase the risk of skin cancer.
Sunburn, heat exhaustion and sunstroke are at the very least unpleasant and can, in fact, be life threatening. Protect yourself with a high factor sunscreen and cover up with shirts and hats, especially during the midday sun between 11am and 3pm. At high altitude, there is less atmosphere to filter out the suns' harmful rays. In hotter and sunnier climates you will also sweat more, so drink more non-alcholic fluids than you normally would, even if you don't feel thirsty and frequently re-apply sunscreen and insect repellents that may have 'sweated off'.
Always buy the highest factor sun screen that you can find. Then just use less of it than you would normally with a lower factor. This will make your bottle go further so saving you valuable space in your bag.