Day sailing requires mostly getting the idea to build a boat, and then finding the right plans. You don't have to worry about long hours at the helm, or inclement weather. You can pick and choose when to be on the water and when to follow the ducks, and get out of the water. Any form of long-term cruising, however, means buckling down to the specifics and trying to think of every possible need before you get the plans to boat building. Overnight cruising for more than a few days with 2, 3, and 4 people on board would need a boat of at least 30 feet in length for comfort and safety.
Your next, very important, consideration is just how much money you have to spend on boat building. Staying between 30 and 40 feet keeps you roughly in the same ball park as far as buying supplies and outfitting your choice. If your eyes are bigger than your pocketbook and you decide you want something above 40 feet, this could easily turn into a twenty-year project, unless you are heir to a windfall.
Once you've determined the size of the boat you wish to build, think of the use. If you want speed, you'll want a long and skinny hull design. Taking the family for long cruises, you'll want comfort, stability, and maneuverability, which means a shorter, fatter, slower hull design. Both boat building designs should move through the water with the least resistance and best stability at all points of sail. After that decision, you can focus on the choices of keel, rigging and interior floor plan. You can play with the interior throughout the building process but not with the keel.
A full-length keel will give you the best ride in a 30' to 40' ocean-going vessel. For those boats 40 feet and above, try a modified keel for a little more boat speed without much loss in stability. In boat building, accept that most all other types of keels have a place on day sailors or racing hulls with a few exceptions. While not yet as popular in the United States, a double keel bottom is very stable and with the shorter draft, you can visit more interesting nooks and crannies within around your anchorage. Rigging is a matter of choice and you can find out what you like best by renting any number of sailboats throughout your boat building period. If you are still unsure, visit your local boating supply store and locate some designers who will be more than happy to help you. Knowing what you want to do, where you want to sail, and how much just another foot or two can cost you when you are building your boat, will be a clue to how long your own boat's berthing process will take.