I usually buy bird toys that are on clearance for less than 5 dollars. I make sure there isn't string, just because I don't want anything bad to happen. I've used cat toys before. The plastic balls with bells inside-- those are hits with babies, apparently. I'm sure some dog toys would work as well. My rats aren't picky.
I have a friend who works at a fabric store and they gave me this gigantic cardboard tube. I don't think it was for just any old fabric... Can't remember what they called it. Even my biggest rats can fit in there, albeit they have to do the caterpillar to squeeze in. Anyways, you could ask fabric stores if they have any extra cardboard tubes they were about to throw out.
I'm re-purposing an old paint tray that had never been used. Going to clean it up and make a little bath/pea diving toy for them.
One idea I read forever ago is you can get an old bin and fill it up with rat- safe soil (presumably organic, no pesticides, etc.), and then plant seeds in the soil. Allegedly rats will dig in the soil, find seeds, and then eat them. I haven't tried it simply because it sounds so darn messy.
I've made quite a few things out of TP tubes and even paper towel tubes. A lot of Pintrest and YouTube results for what you can do with those. I've used paper cups to make foraging toys. I always like those rats who meticulously are trying to figure out how to get into the toy, and then you have the big rat who just brute forces his way into it. That's always a funny sight to see.
You can make toys out of food as well. I've hung up sugar snap pea pods on a string before. I've done the same to a corn husk as well.
If you have any fleece, you can make a "fleece forest" as I call it, mostly because I can't remember the name of it. Hang various strips of fleece up on a number of strings and make a little forest. Apparently rats are well entertained by this. I keep meaning to do it, but just forget every time I bring fleece bin out.
My rule of thumb is if it's kid safe, it's rat safe. I gave my rats some infant toys (like those baby keys they make for a teething baby) and they chewed those up within days. Just make sure whatever you're getting is really cleaned off by yourself, especially if it's from a garage sale, Kijiji, or Craigslist.