We love our 2006 Prius! We have one 10 year old child and she was in a five-point harness seat still when we got the car (forward-facing, though). We've had many different driving situations with carpooling and camping and visiting relatives, etc.
~ Five full-grown women - I was driving and a 300+ pound friend was in the front passenger seat while three ordinary size friends were across the backseat. Everyone was in a seat belt and we had a great time! (I forget where we were going, but at least a 30 minute drive each way...probably further.)
~ Two women + three kids to Girl Scout camp in the mountains - This was the tightest fit. Us adults were up front while the three kids were across the backseat. Two girls were in backless boosters (Monterey+Graco) on the outsides while the largest girl (heaviest, but not BIG) was in the middle. They fit fine, but there was a required order of entry/exit and seatbelt latching. We managed to pack all our belongings for three days and two nights into the hatch even though our friends brought two rather large wheeling suitcases! (This was their first time camping, so we cut them some slack and educated gently.) This type of camp did NOT require cooking equipment. My friend had her backpack purse and my daypack at her feet while the girls had a pillow on someone's lap and a bag of food at the littlest one's feet (her feet didn't touch the floor nor the bag, so it didn't impact her). On the way home, we stopped for pies and cider and those had to be fit in creatively.
~ DH, DD, me, and lots of home repair supplies - DD was in a full-back booster (Monterey) on the 40% side and we had the 60% side down flat. Every inch was filled and we couldn't even see DD from the front seat due to the sheets of something-or-other going across the car directly behind the front seats. DD had plenty of room, but these sheets were taller than she was at the time. We also had long pieces of baseboard molding running the length of the car that I had to duck under to get in and out of the car. It was a short five-mile drive and we were in the midst of major flood repairs and needed to get these items right then. We've actually fit a ton of home repair supplies into the Prius! One time I had to pick up the 16-foot baseboards for our entire house from a place 2 miles away by myself. The guy looked at me and then at my car and back again. We both started laughing and then set about making it work. The boards stuck out the hatch and went through the car (backseat UP) and came out the passenger window and ran alongside the front of the car until they were even with the front bumper. I had a hand towel we keep in the car for cleaning feet at the beach and we wrapped the side view mirror with that and taped the boards to it. I always keep bungee cords in the secret back compartment and we used those to secure the hatch. He tied a red flag to both ends of the baseboards and off I went. A neighbor helped me get them inside.
~ Me + friend + babies - I had three carseats across the backseat plus strollers and diaper bags, etc. It required a rear-facing infant seat in the middle and the widest FF seat on the passenger side (that side is a bit wider in our model due to the battery vent). Lots of configurations work, but creativity and persistence is required. It helps a LOT to have built-in lock-offs on the carseats!!! We had a Britax Roundabout at the time and that fit on the driver's side (opposite side from the vent).
~ DH, DD, me + very large grill - We thought we might have to leave me and DD and her fullback booster (Monterey) at the store while DH took the grill home and then came back to get us. However, the salesguy knew a couple tricks even we didn't know and this was earlier this year! It was very fascinating, but quite safe and everyone and everything fit.
~ Me + friend + food/clothes/linens/scrapbooking supplies for three days in the mountains - This sounds a lot easier than it is! LOL I leave DD's carseat at home and put the backseat down. I get my half of the car (driver's side all the way to the back) and she gets her side. Whatever doesn't fit, stays home! She has a huge SUV and the cost of gas always encourages us to take the Prius despite the space limitations. BTW, we're not just bringing supplies for ourselves, but the organizer assigns each person certain tools for sharing amongst us all. I always bring home all the recyclables from the entire weekend, too. Someone else brings home all the non-recyclable trash.
At no point have I had a large dog in the Prius. However, the space on the floor in the front passenger seat is good as is the floor behind the front seats. Depending on what you are hauling, the hatch could be used. We use our cover as much as possible, but there is a built-in slot for it when it is not in use. Also, with one child, we have mostly kept her carseat or booster on the driver's side, which is the 40% side of the backseat. That way, we could lower the 60% side whenever needed without reinstalling her seat. (They call it a 60/40 split backseat, but it seems like 65/35.) We also keep a tarp in the back and fold or unfold it to cover the space needed. (It covers many needs, too!) I fit DD's bicycle in the back around her and bungee it down. There are metal loops in various spots. There are secret compartments all over the car and we keep a set of bungees in one. My yoga mat is bungeed in a spot all the time to keep it from rolling around. We use the extra space under the top hatch floor to store beach supplies since we live so close to the beach and go spontaneously often. A large freezer bag also fits under there (rolled up) for frozen/cold groceries, but we move it when it is full. We bring our Christmas tree home in the back of the Prius with DD every year. Wrap the tree in the tarp and lower the 60% side of the backseat. Slide it in at an angle.
At one point (a year or two), we had DD in a Safety First Apex 65 carseat and it would only fit on the 60% side of the backseat for our needs (allowing the 40% side to be folded down). That worked for many of the things we normally do. It'd take some research and trial and error to figure out how to fit two carseats on that side together, so that you could use the 40% side to expand the hatch when needed. Also, the space behind the front seats is a surprisingly useful space for certain types of things that would also fit in the hatch, but is faster and easier with kids in carseats and "stuff" in the hatch.
We average 50-55 MPG in the warmer months and 45-50 MPG in the cooler months. Our lifetime of the car average is just over 50 MPG.