I feel like I don’t know what to do to keep my puppy stimulated

To be honest, I didn't know until after getting a puppy that they can't be outside or exercising for more than 5 minutes per month of age. Thankfully I found this out quickly, so no joint damage for my pup. Okay, so we get 15 minutes of exercise per day. No problem. I don't want hip dysplasia or parvo in my pup.

But the thing is, outside is the only thing that really holds her attention. ANY activity I try to establish – tasty chews, searching for treats games, I made a dig box with blankets and layer treats in the blankets, we do obedience training, etc – but it can't hold her attention for more than 5 minutes before she's sitting around the house depressed.

I feel like she starts getting depressed unless I am not constantly engaging her, and I know that puppies require constant engagement, but it feels like nothing I try works. One minute she wants to play, then she wants to solo play, and 5 minutes later she is bored and looking sad. Okay, start a new activity. 5 minutes later she is bored and sad. This might be a bit of an exaggeration but you get the idea. If I give her too much attention she starts acting like she wants to be left alone, but when I leave her alone she starts acting bored and sad.

What I have tried: training games, interactive and treat dispensing toys, puzzle feeders, hiding treats in areas she can chew or dig, etc.

I unfortunately do not have a yard. I did not mind at ALL going outside a lot (I already do) to make up for this, but it seems that isn't actually a viable thing with a puppy. I WILL have a yard when I move in July, thankfully, so this will resolve.

She'll have her final round of shots soon, but it still seems like it's going to be a while before the outside can be a major source of enrichment.

At the end of the day I'm signed up for this and she has been a very good pup, so I don't want to complain. I just feel like when I need to get anything else done, I feel like I'm neglecting my pup because nothing entertains her without me, or sometimes even with me.

Any advice would be helpful. I have a 3.5 month ACD puppy. I was well aware of the enrichment needs of the breed and feel I have tried so much (confidence games, obedience training, puzzle feeders, dig box, flavored and real animal chews, frozen chews, toys that dispense treats with interaction, dual play, and more) and still feel I am not doing a good job. I would like to try scent games next, but part of the problem I am having is any time I spend trying to prepare the next activity, is time I am not engaging her in an activity.

I want to ultimately train her to perform simple jobs around the house (fetch my keys, clean up toys, etc) since she is a breed that needs a lot of mental stimulation, and I am hoping scent training will help me start putting her to work. She is VERY good with obedience training (learns commands in a day or less) but we have yet to move beyond simple commands. I have had her for less than 3 weeks and I feel like she needs something more complicated, but I also have people telling me not to try anything complicated before she has the obedience basics 100% down.

So no advanced training, no outside adventures, nothing I try seems to be a huge hit, so what do I have her do?

Of course I know she needs lots of sleep but in this time I am also busy with addressing my own needs and my job (I work from home so thankfully this does not detract much of my presence, but it needs to be done)