I'm thinking back to 3.5 mos - mostly lying on the floor in A time! Either with a simple wooden baby gym to look at (that wouldn't last long before they wanted it moved) or with things that they could possibly reach for and try to pick up. A teddy just out of reach but in sight, that sort of thing. And a bit of playing with me too, we sang a lot of silly songs, and played a wee bit of peep. But I was careful to give them time to be alone and amuse themselves too.
Creations is completely right about the things they need to practice - at this age they're building up core and other muscles so that they will be eventually able to sit unaided, crawl etc, and so on. Both of mine did a lot of wriggling about on the floor, grabbing feet and trying to eat them
working on coordination of hand and mouth, practising pushing up on the arms, trying to push with the feet, etc. All that picking stuff up and putting in their mouths is preparation for feeding themselves. If you want to help her with physical exertions, what she needs right now is tummy time, not much more. Neither of mine did any sitting, other than on my knee, until 5.5 mos when they were able to sit on their own, quite suddenly!
I'm wondering if the places she is sitting maybe don't allow her much wriggle room - as in, is she strapped in a chair, got something over her etc? I would try letting her stretch out and kick. The next time she says 'nyeh nhey' to the gym thingy, just move it and let her lie there and figure out her next move. Lie down on the floor beside her if you want to, but hang back. Tracy would say 'start as you mean to go on' so try to avoid getting into the habit of constantly entertaining her. I appreciate that's easier said than done, but try to hang back and show her that she can have low-key fun too.