Building a raft can be a fun and educational experience. Unlike boats, which have more complicated designs, rafts are simple to construct, and consist of two basic parts: a flotation system and a platform or seating area. Of course, you will also want to equip your raft with propulsion and steering, which you can accomplish---on all types of rafts---by using paddles. Some of the best ideas for water rafts include using foam, PVC pipe and plastic barrels and/or buckets.
According to Ctriverraftrace.org, the most popular material do-it-yourselfers use for constructing water rafts is foam, such as polystyrene (styrofoam). You can find foam for free in a number of places, including landfills, construction sites, public dumpsters and even in the trash (oftentimes inside of boxes) in front of homes. Of course, you should always ask permission before taking anything off of the property of a company or individual. According to the above source, you could also pay a visit to your local marine supply store or boatyard, and see if there is any second-hand, marine-grade foam up for grabs. In the worst-case scenario, you can always go to a home improvement store and buy sheets of foam.
Once you have your foam, try experimenting with different configurations. One idea is to convert a picnic table into a pontoon boat, which you can accomplish by fastening packed foam under each bench plank.
Another material that you can likely find for free---particularly at construction sites---is PVC (polyvinyl chloride) pipe. According to Boatdesign.net, to build your own PVC catamaran raft, your first need to find several pipes (ideally of the same length; if not, cut them to size) and seal off their ends so that they are airtight. You can use expanded foam to accomplish this. Next, split up the pipes evenly and make two bundles, which you hold together using rope, straps or---as Boatdesign.net recommends---polyester fabric. All you need to do now is attach a suitable platform, like a piece of plywood, to the tops of your two hull bundles.
Plastic Barrels / Buckets
You can also use plastic barrels and buckets in a fashion similar to the PVC piping in order to make a catamaran raft. However, another idea is to construct a Native American canoe-style raft, which you accomplish with the help of a ladder. According to Ctriverraftrace.org, you should use your ladder as a frame, and fasten 5-gallon plastic jugs in between all of its rungs. To increase buoyancy and stability, you will want to add some foam sides to your raft, which you can attach along the ladder's vertical framing beams. For even more stability, try adding an outrigger to your craft, which is a smaller, second hull that is left unoccupied. To do this, simply connect two wood planks from your ladder out to a long chunk of foam or a configuration of plastic barrels and buckets.