Nina Ottosson toys are great, but are meant for interaction between handler and dog too. They are not intended to be used like a stuffed Kong. It helps the dog learn impulse control as they wait for you to fill the toy, then teaches them how to use their noses (or paws, depending on the challenge, and your rules), as well as how to problem solve (sliding covers, lifting bones, pulling drawers, spinning different levels, etc).
I own most of the Nina Ottosson toys (I managed to get a stupid-great deal through my trainer who made a large order for her hall), and do enjoy bringing them out every now and then.
If you are simply looking for something like a kong with a slightly different challenge, I recommend Busy Buddy toys. We like the Tug-a-Jug (the rope smells really bad very quickly! Ditch it when it's gross, and toss in a tennis ball instead), Kibble Nibble, Twist n' Treat, and Magic Mushroom.
They are pricey compared to some toys, but you can get them online for cheap (I know amazon has a small variety of nina ottosson and busy buddy toys).
Or, if your goal is to encourage your dog to problem solve and use his/her senses for real cheap, go to the dollar store!
We have something we've called Eva's "trough". It's a planter that is built to go over a railing, so it has a large barrier running down the center. The two sides are just wide enough to fit her nose. It keeps her occupied for a while.
I know Nina Ottosson has a hiding blanket type thing, which is really just a blanket with sequencially smaller blankets stitched overtop. The idea is to hide kibbles under the folds to encourage the dog to use his/her nose to locate the food. you could just as easily toss a blanket on the floor, and do the same thing without all the stitch-work or money spend.
Our douve is very "fluffy" for lack of a better word. We toss a handful of kibble on the bed and send Eva after it. As soon as she jumps up, all the puffs and folds naturally hide some of the kibbles. Mind you, we only do this on cleaning day, since the douve usually needs to be cleaned afterwards.
There are tons of cheap, easy things you can find around your house, or at the dollar store to keep your dog occuppied. Just get creative!