Are the walls insulated? If not, do that first. Most of the heat escapes there...especially the part nearest and above the ground level.
With a fully insulated wall, you might not even need to insulate the slab. in MN we insulated our walls and then left the concrete floor as-is (stained it). Only on the coldest winter days did we need to turn on the fireplace and when we did, the slab was fine comfort-wise.
Otherwise, the most typical way to insulate the slab will be with foam board. Typically EPS or XPS with a floated plywood subfloor on top.
But if you have no intention of ever finishing the basement, maybe insulating the first floor underside might make more sense.
UPDATE
The above is valid for a house with a basement. I misread your question initially. It appears you have a floating concrete slab as your foundation. To insulate that you need insulation...either under it (preferred) or on top of it. Obviously adding insulation underneath is not practical after the fact.
The cheapest solution is likely just a new floor covering...carpet with a think pad...maybe cork, etc.
More complicated, but doable, would be to lay down EPS or XPS and then a floor on top of that. The trick there is that this will change the height of your floor...likely causing issues with doors, thresholds, cabinets, etc.
You can install an interior weeping system similar to what you would normally do on the outside, but without the waterproofing.
Start by jackhammering out about two feet of floor around the edges of your garage, as close to the walls as you can get. Dig down until you reach dirt, and if that's not at least two feet down, keep going. Now, put down a couple inches of 3/4" crushed gravel, and then install weeping tile (it's not tile anymore; it's 4-6" corrugated plastic drain pipe with a nylon mesh sock over it). Make sure the sock is continuous across any joins you make in the weeping tile for corners etc. This is a good time to direct those drain holes down into this trench, where the water they drain will flow into the weeping tile. You'll also need a sump, or a connection to a downhill storm sewer; direct the weeping tile to this sump pit or the storm drain. While you're doing all this a center drain can't hurt too much; you can tie it into the weeper wherever it's convenient, just make sure the garage floor and the drain flange are level, or that the garage floor slopes slightly to the drain.
Now, you can install waterproof sheathing on the interior walls contacting the concrete. Normally this product is designed to go outside the foundation walls, but in cases where that's infeasible it can work this way too. The idea is to trap moisture that weeps through the wall behind the sheathing, where it will then be directed down the wall and under the slab to the weeper. So, you'll install the sheathing, which should have an air space between wall and most of the layer, and make sure the end is down in the trench by the weeper.
Backfill the rest of the trench with 3/4" crushed gravel up to the slab footing, then patch the slab with new concrete. Your garage should stay much drier.
Best Answer
I'd pour the slab, leaving a 'hole' where the bath will be. Essentially, you will make a hole in the slab using forms.
After the slab sets, remove the forms, come back and pour your slightly-lower slab in the hole that was left.
Ideally, use some rebar in the original pour to tie in the new slab for the bath.
Alternatively, maybe consider pouring it all flat and use a transition strip between the concrete outside the bath and the slightly raised tile.
Alternative option 2: if the whole house is going to be polished concrete, why not the bath too? Maybe stain the bath concrete a different color?