Hi dude, i think that one of the more important steps to "design" an exercise is looking for stuff that we enjoy doing it (and see the "brain benefits" like a bonus), if you look at the tutorials and get more in deep what each networks function "does" you can tailor yourself your own exercises.

Of course in the activity you are explaining in your post, will require attention/concentration skills, plus maybe a little of working memory to "update" it, keeping track of the numbers etc ..

Tip: If it is (for you) too easy, or you get bored fast .. probably is not (for you) the best exercise to do.

I think is cool the idea to "make the best" of our day, because me too for example have a lot of "wasted time" where for example i'm waiting for someone, and i feel like my brain is in "passive mode" doing nothing productive ..

Which i think could be great trying to do stuff like this "outdoors", because theorically we need to "filter" a lot of "noise" (other people talking, sounds that could distract us etc) and that alone can be a challange to focus our mind to do whatever mental exercise you try to do.

For me, the difficult is being selective of which exercises i choose! (too many interesting stuff to try, 24 hs only in a day)

A tip that work for me (if you have the time) if maybe not choose a "simple" exercise that probably will not bring a direct "practicity" in your everydaylife, but for example more like hobbies that not only will train your brain but hopefully enjoy more and open new possibilities in your life, for example learn to play the guitar (N4), you will not only build up that network but gain a great skill that can be used for the rest of your life ... makes sense? that way i can stay months doing it, getting better at it gradually,  and not worrying that i need to change every day or every week my "list of exercises".

Other people maybe enjoy it more doing more "selective/simple" (isolated) exercises for N4, and that's cool too, variety is good.

My theory is that if you choose to learn the guitar (only an example to make my point) in that long process you will still do/practise all that isolated components anyways (because the process of playing the guitar good requieres it).

Probably in the best case you will do both =)

Sorry i wrote a lot, hope that it helped in something.


Cheers