You can't get the dining plan unless you are staying onsite. If you take a day to go to Sea World, but still stay onsite, you'll be losing those meals or trying to play catch-up.
Like others have said, if you look at how ticket prices break down, the difference between a 3 day ticket and a 6 day ticket might only be $20 or less, total! Adding more days after that is even cheaper (like $6 a day for the whole family). Sometimes the price of the 3 day ticket and a 4 day ticket is the same.
If you go to SeaWorld and have a Southwest Airlines frequent flyer card (it's free and you can sign up and print it online), then you can get Sea World tickets at the gate for 20% off. Of all the deals floating around, this was the best deal out there.
We just took a 10 day vacation in Orlando. We spent the first day at Sea World (tickets were around $250 with tax for the four of us -- we had to pay adult prices for our kids. If you buy a one day ticket, you can actually use it to come back one more day for free within 7 days of the first use. We didn't do this though. We stayed in a hotel 1.5 miles from Sea World for 2 nights and paid $33 per night from priceline.com. It was a decent place with a bedroom suite, nice pool, etc... Total for all of this (including park admission, hotel, food, etc..) was $420.00
After Sea World, we went to a Daytona Beach area for 2 nights. We paid $94 per night for our hotel on the beach. We rented chairs and an umbrella for our beach days (about $20 per day). Breakfast at the hotel was under $10 for all 4 of us (eggs, bacon, pancakes, hashbrowns). With other meals, gas, hotel and some shopping we spent around $280.00
Then we stayed onsite at the Wilderness Lodge for 7 nights and 8 park days. We had the dining plan included. Our package price for this was $3300.00 (woods view w/ bunk beds for the kids). We spent $170 on tips for the dining plan and for Mousekeeping (maid service).
We had a rental car for 1/2 of our trip (via priceline) for $160.00.
Our only other expenses were gas, souvenirs, lunch at Downtown Disney one day (we had a $15 voucher and one dining plan credit so all we had to pay for were the kids' meals), and a few out-of-pocket meals we paid for when the kids wanted to eat off the kids' menu (and we saved their dining plan credits for breakfasts that we shared).. The miscellaneous expenses came to $300.00.
We also flew, so that was an additional $950.00.
Grand Total for 10 days for a family of 4 doing all of the above was around $5400.00.
Depending on the time of year you will be traveling, you will probably find package deals out there, so if you plan to stay on-site, you might not want to run out and buy tickets just yet. If you don't do the dining plan and stay off-site which we did last year, our lodging for a 3 bedroom condo 5 minutes from WDW was $850 for 10 nights (at Windsor Hills). You could buy your own food and cook breakfast and dinner in your condo and eat counter service meals at the parks.