Something like
/ip firewall filter
add chain=forward action=log in-interface-list=LAN out-interface-list=WAN connection-state=new,untracked
and you should push it near the start of filter list - exact place really depends on other
chain=forward rules ... definitely above rule that might look like this one:
add action=accept chain=forward comment="defconf: accept established,related, untracked" connection-state=established,related,untracked
The rule will only show connections from inside your LAN towards internet. Depending on topology of your LAN(s) rule to log browsing between LAN(s) hosts would look alike ... some connections won't show as they don't pass router at all (i.e. if server is in the same subnet as client).