Almost anywhere! You should certainly work on establishing lasting professional relationships with your bosses and colleagues. Especially if they are in an industry you hope to work in after you graduate, the people in your office can be an invaluable resource of information and contacts.
There are also professional networking events that you can attend. In these situations, you must be prepared to give a short elevator pitch about yourself and focus on starting meaningful conversations with people. You want to come dressed professionally and have a small case of business cards to give to those you speak with.
You also never know where you may run into contacts that you have made. People have run into their contacts at a restaurant on Saturday night, at the grocery store, and even at a bar. Although D.C. seems like a large city, you will be surprised at how often you run into people you know. Thus, it is important to always be aware of your behavior and actions.