The other suggestions for seats are great, but keep in mind that it's tricky to install a seat in a plastic boat. Nothing bonds well to polyethylene, particularly if it's in a part of the boat where it will get wet and/or dirty often or where water pools, such as the inside of the hull. If you examine your current seats, you'll find that they're screwed to the hull for this very reason. I wouldn't bother trying to install any type of seat that much be glued in. Your best bet would be to try to move one of the current seats to the center of the boat and add some extensions to the sideplates (if necessary) that will reach the gunwale or side of the hull, where you can screw them in place. If you want something lighter and nicer, the Kajak Sport seat is great, but it's also ~$125.