The LAN ports and the wireless connection on your router act like a switch. The switch will have learnt the MAC addresses that are connected to each port. When the switch gets a packet it will route it to the port with the destination MAC address, i.e. it will switch the packet.
There is no real way to monitor all the LAN traffic on the router on one PC. You could run wireshark on all the PC/Laptops on the router if thats what you realy want.
If you want to study what goes on you will need a switch with a mirror port. That will mean a managed switch, or at least a web managed switch. You may be able to get an old CISCO Catalyst switch off ebay quite cheap. Just check that the switch will be quick enough for you. In the UK there are alot of 1900 Catalyst switches for sale but these only have 10 BT ports not 100 BT.
You can setup the CISCO switch so one port can be used to monitor the traffic on other ports. That is what I call a mirror port. I think that is what Cooky meant. I do not think that you have to have the enterprise level software (or switch) to use port mirroring however.
The web managed Dell switches will also allow you to set up a mirror port, and these can be bought for about 80 - 90 pounds in the UK for the 16 port version (Power connect 2716). The 2716 switch also has gigabit ports with 9k jumbo frames.
I think some of the managed 3 com switches also allow you to setup a mirror port.
If you do not want to get a switch then see if you can find an old hub. Some of these had 100 BT ports. Because its a hub not a switch each packet it receives is sent to all the other ports. This means you can monitor all the traffic on that HUB at one point. For monitoring HUBs are good, for actual data communications they are not so good.
Rob.