What is the etiquette about the length of a visit when giving departmental seminars? If you are invited to give a departmental seminar and the department offers to put you up for up to two nights, how long should your visit be? I find it a little rude when our speakers arrive an hour before and leave an hour after the seminar. Do you have to spend a full day at the host department?
Think of this as an opportunity, rather than an obligation. 

Find people in the department who are doing interesting work and try to organise a short meeting with them. Volunteer to hang around after your presentation to talk to PhD students (and ensure that your presentation has some appeal to keep people around). Ideally, try to have a fairly full schedule to maximise the benefits gained from the opportunity, though avoid having every minute planned so that you can have spontaneous extended discussions with people, should the opportunity arise.

Planning ahead is probably key, as not everyone will be able to accept an unannounced visitor for a lengthy discussion.