Hammock
The trick to making hammocks is to make two, so you’ll always have an extra when you throw one in the washer if it gets chewed to shreds.

Take a square of strudy fabric (old jeans or plain t-shirts, fleece, etc)
Hole punch the corners
String strips of extra fabric, rope, or shoelaces through the holes
Secure to the cage with safety pins

For in the winter, make a snuggle hammock:

Cut the fabric into a large rectangle and fold it over so the top is 3/4s covered
Sew the two edges so they can still climb in
Hole punch the corners
String strips of extra fabric, rope, or shoelaces through the holes
Secure to the cage with safety pins

Houses
Stock up on empty tissue boxes, soda boxes, shoe boxes, cereal boxes, etc. These make great homes for rats as they can chew on them and hide in them.
Cardboard soils quickly and the rats will chew holes through it in just a few days, so you’ll want to replace them often–Remember, stock up!

Empty milk or water gallons are great. Cut a big enough hole for your rats to climb in, and string it up from the top of the cage! My rats love climbing in it and making a rat pile.

Chew Toys
Empty toilet paper and paper towel rolls (for extra fun, leave a couple layers of paper on the rolls! The rats will have a blast shredding it and chewing on the rolls)
Nuts in the shell (unsalted, and not often as nuts are high in fat)
Pieces of thick cardboard.

Tunnels
For young rats, toilet paper rolls are the perfect size for them to climb through.
For adult rats, 3 inch PVC piping makes great tunnels. Make a maze with them and hide treats in the tunnels.
Empty coffee creamer containers–cardboard, make sure there’s no silver lining. But off both ends.
Old pants. Cut the legs off and string it up to the cage. Make sure the pants are a big enough size to comfortably fit 2-3 rats at once.