You sound like us a few years ago. In 2001 I packed three kids, baby items, stroller, coolers, and two adults into mini van. We actually did okay.
My first suggestion is to use EVERY available space. As an example, we had two cases of pepsi/mt. dew in the space between DD's seat and the door. Worked out great, as it allowed DD's toy items to be up high enough for her to have and play with without having to take up precious space on the seat. Since DD was only 13 months and in her car seat, we put items that were okay for her to stand on under her feet. Under the seats, I rolled up beach towels, the kids took them out and used them rolled up as pillows and unfolded as blankets. On the back on of seats, I had the hanging things with toys/books/games/etc. We had the largest cooler we could put in up next to the other side of the van between the seat and the door. I bought a tall plastic rubbermade that fit perfectly between the front seats and that was filled with snacks for the kids. I also agree with using smaller bags. They are more flexible than big ones. You can stack them in and around alot easier. We had a small, softsided cooler next to us for drinks while we were driving. We had a bin that fit under the seats for the videos to be used in the TV/VCR. On the seat between the boys, we put a plastic rubbermade bin (just larger than a shoe box). That way we "doubled" that space, and provided them with a table like surface for drawing, coloring or doing homework. We put in the rubbermade the clothes and toiletries we'd need on the drive down there, that way all we had to do was grab that bin when we got out, instead of having to go through all the stuff and repack it. We had every nook and cranny filled. It worked, too. The kids all had more than enough room to sit in and foot room. On the way back, we had room for souvies because we no longer had all the food/soda. It was a bit of a challenge for the boys to get in and out of the back quickly because of the cooler on the side, but they either climbed over it or we took the cooler out and put it back in to let them through.