You mentioned camping in the OP. A day tripping instead of a camping hull for the family narrows down the scope of use. It may allow you to select a shorter, more solo compatible hull.
Adding weight in a hull displaces an equal weight of water as the hull settles lower in the water. The displacement numbers associated with designs represent the weight to settle the hull to a certain waterline, usually 4 inch or sometimes they specify a waterline.
Lets take the Bear Mountain Boat Works Freedom 17.
Freedom 17 linkThey say displacement is 420 lbs
Draft is 4.25 inches
Beam at the waterline is 31.75 inches.
So, with 420 lbs in the boat it will settle 4.25 inches into the lake and at that waterline the width of the hull is 31.78 inches. That is the load the boat was deigned to move most efficiently through the water.
Capacity is a bit different. You can certainly safely add weight beyond 420 lbs to that boat. It might be a tiny bit less efficient, but you would have to put 160 more lbs in there before you would notice it. The BMBW website has capacity numbers, but unfortunately they didn't fit to put them on the plans page. Go to this page for very useful capacity and stability info.
stability and capacity link Read all of this, it is good information, but scroll down to the chart with stability factor and capacity. Note that the Freedom 17 has an optimum capacity of 150-510 lbs. This means that any less than 150 you would have a hard time making the boat go straight and up to 510 it is going to perform well and be safe.
So add up all your weights, through in 50 pounds for cooler and day gear and see what hull suit your needs. Stability factor should be considered, too. Four people in a canoe demands that.