The '1-2-3' Hammock
Hammock Variations
If you want a hammock with a little more staying power, you may want to hem the edges or use grommets and wire to hold it up rather than something that can be chewed.
There are a few different kinds of hammocks as well, including 'snuggle sack' type designs where it is either a triangular tunnel of fabric usually supported by cardboard on the inside for stability or a bag-like design that hangs from the wall or ceiling. Another idea is to make a tube of fabric that extends from one level of the cage to another instead of using a ladder, and the rats can climb up and down it as they please. As with any toy in their cage, make sure you wash it at cage cleaning time or more often if necessary. Heavily chewed or soiled hammocks may need to be replaced, so choose inexpensive materials to begin with.
Whether used for a sleeping area or just for fun, a simple cardboard box can be great fun for rats. Simply find one the right size for your cage, cut off the top flaps, and cut at least one door in the front. You can also make several doors, or some windows or a 'skylight' in the ceiling. Rats like to hide inside, and they will also have fun gnawing it to their liking. When the box becomes too dirty or chewed, throw it away and replace it with a new one.
Nesting boxes can also be made of wood using much the same design, just keep in mind that it will be chewed and urinated on and if it is unfinished the wood will very easily soak up the urine and begin to smell, not to mention it will be a health hazard to your rats.
Rats can sleep happily inside almost anything they will fit in; nesting boxes can also be made from flower pots (plastic or ceramic), PVC piping, bird nesting boxes, a snuggle-sack style hammock (see above), and more.
Tubes and Tunnels
For connecting tubes similar to the commercially available ones, use PVC pipes which fit neatly together. Make sure you choose ones that are wide enough for your rat to comfortably walk in. To help keep it clean inside, as well as add some traction if the pipe is going uphill, drill several small holes along the length of the pipe for the rat to grip onto and let urine and feces fall through. These pipes are chew-resistant and easy to wash clean. Many different connector types are available; you could make one single tunnel for your rats to play in or even build an entire maze-like tunnel town for a fraction of the cost of what a pet store would have charged you. The one downside of PVC piping is that they are not translucent, so you may not be able to tell where your rat is if you make a large maze. If you drill small holes in the top of the pipes as well as the bottom, you should be able to spot your rat more easily so you can get him out if he becomes stuck or you can no longer watch him closely enough for him to be out.
Drainage pipe can be used as well, as it is ribbed on the outside and inside and is easier for rats to grip. It is also very flexible, which can be good or bad depending on what you are wanting to do with it. It can be a bit harder to clean, especially because it is usually black and hard to see urine or feces on it,
Toilet paper or paper towel roll tubes can also be used as a more temporary tunnel, though they are often too small for adult rats. Poster mailers are a larger cardboard tunnel solution, though not as readily available.