what do you save

Nothing. I toss everything. Sometimes including the model itself.

some of the kits ive got where bought just for the box tops
but i save the instructions and spares

I save all the instructions, all spare parts, and depending on the sprue, perhaps some, all, or none of it. It's good to have sprue lying around, as you never know what you'll need it for. I don't tend to save boxes as it's impractical and fairly useless to have all that space taken up by nothing.

Spare parts, instructions and left over decals.

MMMMMMMMMM BOX ART

As I only re-started this hobby last December, and I have a limited amount of time to devote to it, I have plenty of space, so I just drop everything that's left over in the box, and use the box as a base for the completed model. The only exception was my first tank, an Iraqi T-72, which I built for someone else to use in a diorama. I gave him everything: the completed model, the sprues, the extra decals, and the box.