Actually, fetch isn't very interesting, when you talk about mental stimulation. So that may be part of your problem.
We play games that require my dogs to use their brains. One of our favorites is "find it."
Start by tossing it right in front of her and saying "Find it" and when she jumps on top of it, praising her. Then toss it further away (telling her to Find It), a little further away each time, and praising. Then (and you might have to tether her for this unless you have a partner who can hold her or til she learns to hold a stay) go into a room where she can see you walk in and out of, and leave it there. Every time she brings it back to you, it's a huge party. Eventually, you will be able to hide it in any room of the house and she'll be able to find it.
I like toys that squeak because I can Woo-Hoo as soon as I *hear* that she found it even though I've remained in the room that I released her out of.
Pretty soon, you won't be able to hide it in any room of the house (or out in the yard) because she'll find it nearly instantly. You should see me digging up the leaves in my back yard, hiding my GSD's favorite orange cuz under pots, and he always finds within a couple of minutes. But we keep playing. It's one of our favorite games. At some point (when he was about a year old, probably), this game became more challenging for me. But we love it!
We also play hide and seek. But he finds us, literally, in seconds. The only place that he didn't find Dh within a few seconds was when Dh climbed on the roof of our house. Still, it's fun. And we all get our exercise running around our house. And our dogs use their brains. We will go into our woods (when we know no one will be around) and play hide and seek too. That's mentally stimulating as well.
I also trained my dog to play soccer. I gave him a soccer ball, which he almost instantly popped carrying it around. That's ok. I then rolled another in front of him. When he batted at it which his paws or with his muzzle (still carrying around the deflated ball in his mouth), I told him "Dribble!" and I kicked around the ball with him. If he dropped the deflated ball, I urged him to pick it back up (so he didn't try to pick up the "good" ball. The deflated ball filled his mouth so he wouldn't attack the new ball).
The more he pawed and kicked around the good ball, the more I kicked it back to him. Pretty soon, he figure out that this was a GAME, and it was interactive with ME (or Dh). Then he learned that he didn't need to bite the good ball (I substituted a smaller toy like a cuz or tennis ball for the popped ball after a while). Then he figured out that he could have fun dribbling the ball around by himself.
He still prefers a wicked competitive soccer match with Dh and me to pretty much anything (including food). But when he's bored, we have soccer balls outside and one in the house, and he'll pick up a Cuz and start dribbling his soccer ball around on his own. Now, our puppy is trying to figure out this whole soccer thing on her own as well.
Take obedience classes, or agility classes. Or just buy a DVD or book and work on training at home. I do 20-40 minutes of training with my kids EVERY day. I shout out commands and they do them as fast as possible. The one who does it the fastest gets the treat first. They get really competitive and move quickly. Since you only have one dog, you can do the same thing by stacking commands: "Sit, down, stand!" Then offer a treat. (I do this too.) Mix up the order. Sometimes treat after one command, sometimes after five. She never knows, so she'll listen attentively and move quickly. Her brain will be working like crazy.
Also, train tricks. Tricks are fun. You'll love them. You'll love showing them off. And you can add them to the rapid-fire sessions that I mentioned above. "Sit, down, rollover, sit, shake, speak!" Hooray! (My younger kids LOVE any rotation that ends with a big bark of SPEAK!
The thing is, you don't have to play for hours. If you play with your dog in mentally challenging ways, that will tire her out MORE than just tossing a ball. She'll be more content and be more willing to chill around the house the rest of the time.