OpenDNS on the router is a start. If there are any open wifi networks that do not go through your router available in the office try to have them blocked.
You can install something like WebChaver on each computer to monitor what each person does.
An admin can often watch any computer as well using something like Remote Desktop.
Having all computers face the open office can be helpful but is not always practical, especially if you have some employees dealing with sensitive data that others shouldn’t be seeing. In that case, you can try to arrange the office in a way that at least one other employee can see that screen. All depends on the layout of the office of course.