Holidays - taking time off work - can be a pain in the butt. Especially if the knowledge that you could be doing something productive with your time is nagging you throughout the entire vacation. If this is a problem you have, worry not, for I have a solution.
In order to make your holiday something that doesn't make you uneasy and unable to relax, do everything in your power to arrange your holiday so that you're not even supposed to relax. For example, visit a country you haven't visited before and the language of which you don't speak. Instead of trying to relax and wondering if you could be doing something more productive with your time, you'll have a handful just trying to get by - finding your hotel, getting food, finding places and going around in the public transport, all awhile trying to speak the language.
You'll be so stressed out that the thought of doing work never comes to your mind.
So, in a nutshell, I'm suggesting that you're not even supposed to have fun or relax during your holiday. I'm saying you should make your holiday a straining experience, such a going-outside-your-comfort-zone event that you'll be glad to return to work after the vacation. It'll feel like you can finally relax once you get back to work.
It might sound crazy, but give it a shot. Try to make your vacation as hard as you can to execute. See what happens.