The marginal cost of extra days on a multi-day ticket becomes very low past 3 or 4 days - about $10/day. So, if you have the available vacation time, I think it makes more sense to get park tickets for every day you will be at Disney, even if you build in some days of only going to the parks for a short time and relaxing at the resort for the rest of the day. ie, if you're paying for 6 nights of hotel, I would get 6 or 7 day tickets even if you know that all you will do on arrival day is go see the evening parade and fireworks, and that on two of the days you will spend only the morning in the parks and then go swimming. To me, if I'm already paying for a hotel, I'm definitely willing to pay an extra $10/day for those extra park visits.
That being said, you will never be able to "do it all," so you need to decide what you do want to make sure to include.
I have seen the site "Your First Visit" (
http://yourfirstvisit.net/) recommended if you're aiming for a "do Disney once and only once" kind of trip. I haven't tried their plans myself, though.