I love Berlin. I was initially only going to stay for 3 days but ended up staying 5 days. It's a giant city so depending on how long you are staying and what you are interested in, your itinerary will look very different. Since it's a solo trip, I'd recommend some free walking tours when you first get there to get a taste of which areas you want to explore and even meet some new friends. I did a graffiti walking tour that was really fun. I stayed at the Grand Hotel Hostel and loved it. The rooms were beautiful and it was super easy to make friends. They give you a free beer at check in and will go over Berlin with you and make suggestions base on your interests. If you like night life, definitely check out some clubs because it's just such a different experience than the typical vegas style night clubs. If you are interested in the major touristy stops (museums, reichstag etc) look online for reserving in advance because the day of lines can get super long.