@NaughtyBoy
Nice TY!
I'm trying to find out if that mod actually raises the Characters stats or if it is just for Roleplaying.
Waiting for the author to answer on the accoring mod comments section.

I recently started a new storymode playthrough and i wondered if there could be another way to raise stats like Strength and Stamina.
Vespucci Beach has a lots of workout possibilities and there's even a mod that allows a character to do the workout animation (but why not raise the stats while doing it?).
There could even be sandbags like GTA San Andreas had, as an alternative to Yoga or Tennis.
(I just cant see the three protagonists as kinds of ppl. doing yoga or playing tennis...)

So Dumbbell for example for Strength and Sandbag for Stamina would be perfect, at least as an alternative.
I always saw Yoga as something Michael is forced to do but he'd rather not doing it. Sandbag on the other hand i can well imagine.

So what i'm looking for is a Mod that actually raises the Strength or Stamina skill by working out, NOT the actual character model (which doesn't change if stats raise anyways).

@princegaz
Thank you for the Reply and the Links!
Very appreciated!
The thing is, i do not want to use trainers that basically allow me to do anything. I want to work and accomplish the things i can buy. Using a trainer makes playing a game completely pointless to me.
Additionally having to change one of the protagonists into a custom character each time i start the game is also very immersion breaking and makes playing again pointless.

Maybe see it that way: i have installed
"Premium Deluxe Motorsport Car Dealership" and "Mors Mutual Insurance - Single Player" altough i could simply spawn the cars i want. But that wouldn't be the same.

Maybe i'm the only one thinking like that i don't know.
What i'm looking for is "simply" a mod that lets me play online mode completely offline for myself, and play some mission mods.
Heck i even don't mind the grind, and do import/Export 200 times or more. But at least there's something to do, something to accomplish.
(i don't even dare to ask for the missions and Heists to be single player friendly, lol)

I actually thinking about playing online mode again (in solo public sessions) but it's annoying since there's always the chance someone else joins suddenly wihtout me knowing. (i just want to play in peace, without getting almost a heartattack each time.)

Anyways thanks for your response. Very appreciated!

Would it be possible to create a mod that lets you create a Character just like you can do in Online gameplay and then use it exclusively in Singleplayer mode (without the story missions)?

Please don't get me wrong, i'm not asking to use it in Online gameplay or have a original character do Story missions or appear in Story related cutscenes.
I'd just love to do a bit of RP in singleplayer where i could do various stuff, like starting a business and so some missions (like VIP missions or even do friends & freaks missions.)
I know there are some mods that let you play (sort of) online content, but instead of playing Michael, Franklin or Trevor i'd love to play a character of my own creation.
So my question is, is it even possible to make such a mod and would it be a lot of work to create it?

Thank you in advance!

@MaxLuk
You mean the VIP missions (sightseer, hostile takeover and the assassin mission) you can do solo in freeroam?
I hope someone competent will make these missions for SP someday.
That would be really nice!