do you FMQ? if so, how about a meander that you then backtrack to make into ribbon?

It depends on how big the open areas are, but if they are small enough that you don't have to actually cross them with quilting to hold it together, I'd outline quilt.
If she just wants the shapes, I would do an outline stitch, 1/4" around everything. It wouldn't be too time consuming....follow up the underneath of one leg, pivot and go down the next leg, stitch in the ditch to hide your stitches going across to get to the next leg and start up again.
I'd use a ruler for it if you're familiar with that technique and a lightweight thread that matched the background. I think it would quilt up nicely.

Watson

Ask your sister what she wants, show her some examples, there are hundreds of quilt photos on Pinterest. She told you what she doesn't want, but that really is no help to you. She probably doesn't know what she wants so the photos will help her find out if she likes modern designs, folk art, traditional, etc.

I would stabilise the quilt by SITD along side the tree trucks which go diagonally.
Then only quilt the white areas with a simple infill of lines going parallel to what would be considered the tree trunk. The parallel lines be parallel to the SITD Lines. This infill would give definition to the branches by making the coloured fabric stand out.

I would most likely stipple it too. This, giving it a nice finish without drawing attention to any single area.

For starters, I would SITD the tree trunks and branches. Then think about some infill in the background.

Jim

I was thinking pretty much the same ting. It is so geometric that a curvy design wouldn't look good.

I agree with thimblebug. This way you would not lose the look of the blocks. Very pretty quilt.