Sometimes I don't file a flight plan, but just use the GPS "direct to" and fly. However, sometimes I'd like to just let ATC direct me to an airport and an appropriate ILS. Exactly how do you do a flight plan en route, with just an "arrival airport"?

You can't make a flightplan without a departure airport as far as I know. Just enter your departure apt (even though you've already departed) but don't select Place Aircraft at Starting Point or whatever it says.David