Mental stimulation is anything that requires the dog to think or use his senses. So training, going for walks where he is able to sniff and explore, any outing can be very stimulating, playing hide and seek either with yourself or a toy/treat as the hider, eating meals out of toys, making puzzle toys where he has to try and get his toys out of a box or a bigger toy, playing games that require him to exercise a little self control (play tug and have him periodically sit, down, or drop it if knows that), and I'm sure there are many others. Long lasting chews are helpful for occupying dogs, we wouldn't have survived my dog's puppy-hood without bully sticks but those don't last as long for every dog. Himalayan chews and antlers are usually longer lasting.
As for agility, if you really want to get into it and perhaps compete someday I suggest you do get started now. You can't do obstacles yet but there is a LOT of foundation work you can do, things that help with self control, balance, body awareness, and focus. These things not only help your dog do better on the agility field but help keep him safer so he's not falling off of obstacles or jumping over contacts.
Martini's ranch has lots of videos documenting how they raised one of their agility dogs, including these puppy foundation videos that are really good. They don't go into a ton of instruction on how to teach everything but give lots of good ideas on what you can teach.
https://www.youtube.com/watch?v=dUmHNOp ... D2405A46C0
https://www.youtube.com/watch?v=rk4DW0u ... D2405A46C0
But she also has "Zelda's Video Diary" documenting her pup's training starting from 8 weeks (keep in mind her Italian Greyhound is going to mature faster than your dog physically so can probably do obstacle stuff sooner).
https://www.youtube.com/playlist?list=P ... D2405A46C0
https://www.youtube.com/playlist?list=P ... 94989C9A04
and I have been working hard on her training though and she knows sit, stay, down, she even dose give paw. I am working on come and mainly leave it which is her weak spot when food is involved 
. the main thing I would be really greatful for is if you could help me understand Mental stimulation better as she is getting bored very quickly. I would love to do Agility with her as I think she will love it but I cant start that until well over a year old so what else counts as metal stimulation to occupie her mind. i am also not interested in any other dog sports.
