Ive hardly bought any toys cus Im trying to save so Ive given them lots of boxes and things the hide food in and for them to chew. Im going to get them a big wheel soon cus Poppy has grown out of the hamster wheel.
Most of my most successful toys are dollar store bargains! I find they like non-pet stuff more. Whiffle and natural wicker balls, baskets to climb & sleep in, cardboard boxes & tubes, baby toys hung up, wood blocks to chew & destroy, appropriate-sized baby and kid toys like dollhouses and playsets. Walk around dollar & thrift stores and be creative... There's so many things that would be fun.
Also, bird toys & natural perches seem a hit more then many small animal items. Same with cat and dog toys.
Ferret toys work well, as do some toys marketed for rabbits.
I recently bought one of those ferret balls that has lots of holes in it for them to jump in & out of, and the little babies *love* it. They chase each other around & thru it, and then roll it around a little bit each time. Totally freaking adorable!
I bought it at Fleet Farm, and it was marketed as a dog toy. It had an inflatable ball inside that I removed.
Also, I like to hang string and fabric strips from my cage. Young rats will bat at the strings, and the older ones will rip them down and drag them back to the nest.
Hammocks are good... for beds and cozy sleeping places, and also for making extra levels. I would suggest making them, if you can - it is much cheaper that way.
You can look in the rabbit/ferret/parrot section for ideas on toys. Just be careful to avoid anything with pine or cedar, and you should be fine. With toys, I think rotation is the key. Rats get bored of the same toys very quickly, so changing them often is a good way to go.
Aside from store bought toys, you can give them nuts (pecans, walnuts, hazelnuts, almonds) for something to chew on. I have also given ours toilet roll tubes and brown paper bags. They love to chew on them and crawl into them, and our girls never get tired of making lots of noise jumping on the paper bags. You can use tumble dryer hoses for tunnels, or pvc pipes. Anything really that won't hurt them if they chew it.
I get kitten toys for my kids they love tearing apart the ball part and carrying around the little bell. They also get most of the McDonald's happy meal toys that my kids are forbidden to have (they always fight over who's is who's)