If you are starting out of Manila you could disobey or put off your standing orders and try for the Japanese invasion fleet carrying the 14th Army from the Pescadores. It usually shows up in or near Lingayen Gulf in Northern Luzon in the first few days of the war. Another option would be the strait between Formosa and the Chinese mainland. It's mostly shallow water so you should exercise caution.
In the Celebes Sea you might patrol off Davao in Mindanao. That was another early Japanese objective, although I don't know if RSRD covers that operation.
Another option would be to return to the stock game. Not very realistic in it's shipping routes, but it was rich in targets if I recall.
