The Marine Liner is a nice, comfortable ride with nice views. I wouldn't have done it as a day trip as I wanted more than a day in Ritsurin-Koen for photography. I got a small book out of that trip. For a single visit, I think you could do it from Okayama. Just plan on 2-1/2 to 3 hours on the train round trip and a full day. In that case, I'd splurge on cabs to get from the station to the park and back. Not the cheapest way but definitely the fastest. Just know that the garden is huge and if you visit the handicrafts exhibit and some of the other displays, it will take up a good part of the day. The ticket booth at the entrance has maps of the park in many languages, including English. When I was there, I had to exchange what they gave me at first as it was in German. The volunteer couldn't tell the difference.

If you decide to base yourself in Okayama for three days, I'd suggest a day for the Takayama excursion, one for nearby Kurashiki with an evening back in Okayama and one day actually in Okayama, seeing Kora-Kuen garden and Okayama Castle (Black Crow) along with trying some of the nice restaurants. If you do Kora-Kuen in the morning there are several little local lunch spots just outside the river entrance for lunch on the way to the river bridge over to Okayama-jo. I am a soba addict and enjoyed what I found there a lot. Ramen, udon and other dishes are also available. Look out for the swan pedal-boats on the river below.

I'd still suggest just taking the train back to Tokyo the night before your flight home and do an overnight at one of the airport area hotels. Eat dinner in Tokyo on the way or a bento on the train, before going out to Narita. I would not take a domestic flight from Osaka (ITM or KIX) as neither is very convenient to the downtown area. Add in the bus or train to the airport to the early arrival for a flight and the train would be virtually as fast as flying but more comfortable than Y on a Japanese domestic carrier. That is my preferred method of travel and a nice eki-ben meal on the train is a favorite experience while in Japan.

If you follow my suggestions, then the 7-day Rail Pass would definitely make sense for you. If you insist on flying back to NRT or HND, then the Rail Pass would not pencil out and you would do better to just pay as you go.