Congrats on your PPL. You are going to get a lot of advice on this one so I will keep it short.
First, fly, fly, fly....then fly some more. In other words, gain as much experience as you can, even if it is just going around the pattern. But don't forget to stay proficient in things like slow flight.
Second, before even thinking of the backcountry, get some instruction. For a low time pilot with no backcountry experience, spend the money and get to one of the mountain flying clinics done up in Idaho.
Also start doing your homework. Here a thread that refers to some lite reading for the soon to come long winter nights.
phpBB2/viewtopic.php?t=1397&highlight=flying+bible
As for the seaplane question, I'm not rated, but from what I gather, it is hard to find a place to rent one after you get your rating. So unless you own one, it will be hard to stay current. AOPA's Pilot had an article on this just a month or so ago.
By the way, if you aren't an AOPA member, join. And do the automatic renewal program so you don't have to deal with the "should I or shouldn't I renew" dilemma every year. It's a great organization with some great benefits.
Good luck!