Rhoda - what kind of packer are you? A light-weight/bare essentials type? or "I better bring it because I would hate to be without it" type? What kind of luggage do you use? Mostly duffles, or mostly suitcases that have a finite size? (In other words, you just can't make 'em any smaller or a different shape.)
If you get around fine in a Taurus at home, consider a full-size car, and use luggage you can squish into spaces. After all, how will you be getting to the airport from home? The luggage will have to make that trip too.
If you're just going back and forth to the parks, do you really need to pay all that extra for a mini-van? Take a look at Premium rental fees, too - they might give you the space you need, and be less expensive than a mini-van.
On the other hand, sometimes it isn't a whole lot extra for a minivan, then it might be worth it. Sometimes you will come upon a special offer. Sign up for all of the email offer programs, and keep an eye on Mousesavers, too.
Best wishes for a fun time - Maddle