I have a smart, high energy dog as well. At 6 years old he's still full of life, gets bored easily, and wants something to do. As a puppy and young dog he was quite the handful. He did go through a 'fake us out at the door stage too. He just wanted our attention and learned that while we might ignore him when he wanted to play we always got up when he gave the 'need to go out and
pee bark'. We got him to stop my combining giving him more attention other times and selectively ignoring he when he asked to go out. He got one chance to go out and pee. If he didn't actually pee then he'd be ignored for the next several time he asked to go out. When we did let him out we'd just stand in the yard with him until he peed, no playing, running around, or anything else fun or interesting...strictly a pee break. Then we'd do something fun with him or give him a treat, or a bit of attention later when he wasn't barking at the door.
I pretty much managed him with a combination of exercise and training. These dogs really do need lots of exercise so it would be best for everyone involved if you could find a way to give it to her. Lots of fetching is a good way to exercise the dog without you having to do a lot of walking. I've never done the treadmill but have several friends that have and so I know it can work really well for some dogs. Is there any way you can have a friend take her for a walk every day or hire a dog walker to come by a few times a week. Maybe a kid that would want to do it for a little spending money? Does she like to swim? Do you have a lake or
pond where you could take her. Swimming really seems to wear them out more than just walking.
The other key is mental exercise. Lots of training helps. I do lots of short training sessions though out the day with my boy. Lots of just practicing simple things like sit, stay, come, lay down. Teach her to 'settle' on her bed or a mat or small area rug. When you tell her to settle she has to go to her spot and stay there. Then call her back to you and give her a bit of attention. Then ask her to go settle again. make the amount of time she has to stay 'settled' slowly increase starting with just few seconds or mins. Once she's worked up to being able to settle for a good period of time this can be a great way to keep her busy while you're doing something else. Send her to settle for 5-10 mins and then call her to you and give her a min of play time, repeat over and over again. She'll settle but keep alert for your command to come back so she's mentally busy. You can do this when watching TV, make her settle while you're watch but when a commercial comes on call her to you and play until the show comes back on then send her back to settle.
Another game that I pay with mine is hide and seek. Take a water bottle and poke a few holes in it so the scent can get out and put a treat or handful of food in it. Hide somewhere while your dog is sitting out of sight in another room. Then call her into the room and tell her to "Find the treat". At 1st just lay the bottle somewhere out in the open, point to it when you tell her to find, ask her to pick it up and bring it to you. When she gives it to you open the bottle and let her have the treats. As she figures out the game you can start hiding the bottle in more and more difficult to find places. You can also also hide multiple bottles around the house in different rooms and she'll stay busy for quite a while finding them all and bringing them to you to open.
Teach her to find and bring you all types of things can also be fun. Teach her to get her 'ball' or get your 'slippers' or really anything you want. Smart dogs will learn the names of objects and be able to bring you what you ask for.
Try to teach her a new command or trick every week. Learning something new is great mental stimulation and can actually wear some dogs out even more than physical exercise. It doesn't have to be complicated anything you can think of or get a book of training tips and tricks. I've done all kinds of activities with mine from formal hunting and retrieving training, to agility, and obedience classes. Meals are a great time to give them attention and training. Mine have to sit and wait until I tell them before they can eat. When they we younger I would ask them to do a command and then give them a handful of food as a reward. Then another command and another handful of food. I'd do this until their whole dinner ration was gone. It would take 10-15 mins to
feed them but they got quality training time in the process.
Treat toys and food dispensing toys can be a good distraction and way to keep her occupied.