If your smaller projects call for clear pine without any knots, you can save money by buying a lesser grade, then cutting out what you need from the clear areas between the knots. Also, when buying wood, remember that face grain is not critical for structural parts which won't show. You can likewise save on lumber costs if you can use shorter or narrower boards, or if you only need one good side of lumber for your project.
If your planning a backyard wetland, locate it where it is unlikely to attract unattended children. Check local safety ordinances and building ordinances for restrictions and permits.