The most important thing when choosing any pet toy is that it is safe. Anything you give your rat must be non toxic, and you should also watch bits that might be swallowed and cause a blockage of the digestive tract. This includes threads off of fabric items and ropes. Also, loose threads might become wrapped around toes or pose a strangulation risk (fleece is a good choice as it avoids the loose thread problem).
Cardboard boxes and rolls from paper towels, toilet paper, and other rolls make good toys. Boxes are great for hiding in, though they will often be shredded fairly quickly (but that is fun too).
Another great idea is to take a variety of boxes and other items to create a rat playhouse for enjoyment outside of the cage during playtime. You can tape together a bunch of boxes and create a network of rooms connected by doors, ramps, bridges and ladders. See an example at The Dapper Rat.
Some concerns have been raised about potential toxicity of the ink and glues used in cardboard and paper towel roll cores. Little data is available on the safety of these; I think these items are safe in moderation, but efforts should be made to use plain cardboard or paper whenever possible.
Most rats love shredding paper. Small plain brown paper lunch bags are great for playing in as well as shredding. Crumpled up paper makes a fun, if temporary, ball. Your rats will likely love digging, diving, and hiding in a plastic bin or box filled with crumpled or shredded paper. Paper towels and tissues are also great for shredding, and your rats can make a nice bed out of these too.
Also, try wrapping or folding a piece of paper in layers around a favorite treat; shredding and unwrapping the treat will keep your rat busy for a while.
Wood is good for chewing (and rats need to chew on to keep their teeth in good condition). Make sure wood is untreated, not painted, and non-toxic. Branches from apple or willow trees are good as well (make sure no pesticides have been used).
Hard-shelled nuts provide good chewing opportunities as well as a tasty treat inside (use sparingly as many nuts are high in fats).
Most rats love a digging box. Take a small cat litter pan or other shallow plastic box and fill it halfway with plain sterilized potting soil (not treated with any chemicals or fertilizer, and with no additives like vermiculite). Plant some birdseed or wheat grass and water it for a while to let the seeds sprout and grow for a bit, then let your rats go crazy in the box. To minimize the mess, don't water the box for a day or two before offering it to your rats, and place it a bathtub or spread a tablecloth or newspapers around the box to contain the mess. Your rats will love to dig in the soil and snack on the sprouts or unsprouted seeds.
Tubes and Tunnels
PVC pipe is pretty indestructible, and comes in a variety of sizes and configurations at your local hardware store. You can get a simple straight piece or use a variety of connectors to create a network of tubes. Choose a size you are sure your rats will be able to fit through without getting stuck.
If you are handy with a sewing machine, you can also make great collapsible tubes out of fleece or other sturdy fabrics. You can even sew a ring cut from a plastic bottle or wide cardboard tube into the ends to help hold it open. Sleeves cut off old sweatshirts are also handy tubes/sleep sacks.
Other Containers For Hiding and Climbing
Clean jars and clay plant pots placed on their sides make neat hiding spots for rats. Mini stacking bins (like those meant for office or workshop supplies) make great hiding spots too. Washed coconut shells are also good for hiding, climbing and chewing.
Rats are very agile and like to climb on ropes. You can make little rope ladders and bridges for in the cage. Cotton rope comes in some nice thick widths and is usually available at hardware or horse supply stores. Just make sure the configuration of ropes doesn't post a strangulation risk and that your rats are not unraveling the threads.
If you are really creative, you can get supplies (such as vegetable tanned leather or other bird toy parts) and make your own elaborate toys.
More Great Ideas: see The Dapper Rat for more interesting and fun ideas to keep your rats busy.