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My Enrico Benetti rucksack. This is the perfect size for a day trip. It has a chest strap and a waist strap. It can expand in the centre a bit, has a great spot for a small laptop or iPad. It is padded in the shoulders, upper back, and lower back, and has two mesh cup holders. It is small enough so it fits in all the lockers at museums.
This waterbottle. It is a Bobble Water Bottle and is wonderful because I can fill up from any water fountain or sink and know that my water is being filtered! Plus, most places in Europe now charge for water and since that is all I drink besides wine it is good to carry my own. Let me address the water thing real quick: most places here when you order water will bring you a bottle, I hate paying for water, especially when it is the same cost as wine or coke, you can ask for tap water and they aren't supposed to turn you down but they do-often. They will say it isn't "hygenic" or "they don't do that here" and if you make a stink they will bring you tap water, but with this bottle (that I carry everywhere with me) it makes it easy to just not have to deal with the water situation.
This scarf. I got it during our trip to Paris last year, it is light but when tied just right and combined with a jacket (heavy or light) it really keeps your neck quite warm, but it is also convenient in the event you come across a place in the summer while wearing a tank top and need your shoulders covered you can use it to cover them. Or if you are wearing shorter shorts and need something that goes to your knees you could easily create a skirt out of it. Seems strange, but it really is a versatile piece for any season so make for a good investment piece.
This charger. I got this at a bookstore chain here for a measley €19. The charge you get from it is crap and it loses charge fairly quickly, but, when you charge it it has enough power to charge your phone to full power. Without a power cord. Without sunglight. You just have to remember to charge it beforehand. Convenient if you just used your GPS for an entire day in a foreign city and want to keep in touch with your husband so he knows you made your 3 train connections ;)
What are your top travel essentials? Anything I should add to my list?