To log all of the information you want, you would need two log rules. One to log the data from wan to router, and a second to log the packet from router to LAN host.
In other words, As the packet passes through your routing tables, the destination will be re-written. If I understand correctly, you want to see that packet's information before it's rewritten, and after (so you can see which host its going to).
The rules may look something like this:
Existing rule which shows wan to router:
iptables -I INPUT -m state --state NEW -j LOG --log-prefix " New Incoming Packet"
New Additional Rule:
iptables -I FORWARD -d LAN_HOST_IPADDR -m state --state NEW -j LOG --log-prefix " [>] NEW FORWARD"
Or for extra credit, and to keep things a little cleaner, create a new chain for traffic forwarded to the LAN HOST, something like this:
iptables -t nat -N forward_to_mypc
iptables -t nat -A forward_to_mypc -m state --state NEW -j LOG --log-prefix " [>] New Forward"
iptables -t nat -A forward_to_mypc -j DNAT --to <address_of_mypc>
Then use the new chain like this:
iptables -t nat -i PREROUTING -i <WANADAPTER> -p tcp --dport 3389 -j forward_to_mypc
That would forward any port 3389 tcp packets coming in the wan adapter, to your LAN pc, and if the packet is new, it would get logged.