Nothing is worse than spending hours figuring out where you want to eat or how to get there. Google map directions to several places and screen shot them. Make a google map with your hostel and all the attractions you want to see. Then you can decide where you want to go first or where a good place to meet up is. This also will save you from walking all the way to a site just to find out it's under construction or costs 100 euros. Also, as a food snob it prevents you from spending money on a crappy meal. Figure out logistics. How much money will you need for transportation from the airport to your hostel? What about bus tickets or a metro pass?
Travel with different people.
Remember this is your experience and no one should take away from what you want to see and get out of the experience. If you think someone is holding you back, let them go and do their own thing, don't hold each other back just because you want to do different things. Maybe your best friend wants to go to a soccer game and spend 150 euros while you could care less about sports and just want to buy a 30 euro drink at a fancy bar. Don't spend the money if you don't want to! People budget their money and time differently. This doesn't mean you should never travel with your best friend, but you might not necessarily travel best with them and traveling (like living) with someone really lets you see a whole person, which may be a good or bad thing. When you travel with different people you are able to have different experiences and especially early on, you'll learn who you travel best with.
Don't plan your trips too far in advance.
Sometimes an opportunity will come up to travel to a place you don't have on your initial list. Sometimes the spontaneous trips are the best ones. Leave a couple weekends open in case you decide to do one because changing flights and trains is expensive and complicated.
Some of the most beautiful places weren't the touristy areas or the giant monuments in big cities. I loved going to North Berwick, a tiny beach town near Edinburgh. It's nice to get away from the crowds and see where locals go. Either look something up before or ask your hostel receptionist or waitress. Many locals are more than willing to recommend their favorite spots and relieved that people want to see more than the cookie cutter attractions.
Get to know the streets and landmarks. Get lost. Find different ways to get back home. Help a tourist find their way around. Make the city your home. You'll feel safer and more comfortable and locals will treat you with more respect if you act like you know what you're doing. Find a place to read, to sun bathe, to throw a ball around. Get outside, even if it's raining.
Take pictures.
Let yourself be a tourist. Take pictures at the beginning and the end of your trip to see how your city has changed and too see how you and your friends have changed. Maybe you don't get on Facebook much, but your family and friends will want to see photos when you get home so you have something to point to when you are telling your stories. Even if you snap them all on your iphone, some pictures are better than none.
Try rabbit or wild boar or whatever the local delicacy is. Maybe you'll love it maybe it will be the most repulsive thing you've ever tasted. Stay away from American fast food; you're in Europe. Eat the food of that country or city! Learn to cook. You'll save a ton of money and find out what you like best and maybe make some friends if you share.
Relax, don't get too stressed out and find someone to talk to or another outlet because with the time difference or lack of phone service or sketchy wifi sometimes it's hard to keep in touch with your best friends from home.
Have fun. You're about to have the experience of a lifetime.
