There are a lot of tools available for monitoring network activity, so it's hard to point to one and say, use THAT.
But if you want a quick and dirty, cheap solution, you might consider using OpenDNS. It's a DNS server w/ added features, which include content filtering and reporting. You override your own router's DNS servers w/ the OpenDNS servers so that all name resolution goes through them. OpenDNS then keeps stats on your activity (i.e., where and when ppl are going). It can even provide various forms of content filtering based on predefined levels (none, low, medium, etc.), or you can add your own domains (i.e., specific places you want off limits).
It's not perfect, and it only prevents name resolution, so anyone who uses an explicit IP address can circumvent it (since name resolution isn't required in that case), but it's better than nothing, and it's start, and the basic features are free (like most things, you can pay for more features, like tracking more content and keeping the stats longer). Nice thing too, it doesn’t require installation of anything, it all runs over OpenDNS’s own system. So it probably won’t raise any suspicions by the end user.
Again, this isn’t a fullproof, intensive, content filtering and monitoring solution. You’re not going to get things like activity replay, or snapshots of their display, etc. Just some very basic content filtering and stats. If it proves inadequate, you can always pursue more aggressive ($$$) solutions.
just after i opened the OPENDNS web, i am not sure it gives me the monitoring of the activities (i.e. reporting which sites are visited), i just want a program that monitors rather than block. could you confirm that this OpenDNS serves that as well?