Most students desire the fact that someone is interested in what they are doing. Honest inquiries and discussion will do much to further the parent student relationship - but don't be too nosey. Remember that your student will be basically the same person you sent away to school, aside from some interest changes and personality revisions. You're providing a real service as a sympathetic ear or verbal punching bag. Sometimes people just need to talk things through to help them figure out their feelings.
Effective communication is rooted in knowing how and when to just listen.
Remember that spur of the moment visits are often unappreciated as students sometimes feel like they are being checked up on. University is a time for exploration. Let your student find out what they want and on their own terms. The best thing a parent can be is supportive of decisions that their child wants to make. Life is one big learning experience.