Have a hardwood floor that has some replacement boards in it, in a bathroom. There are some good size cracks between the boards. What would be the best product to use to put between the cracks and level up the floor so I can put down "peel and stick" tile squares on the bathroom floor. Want it to be completely level and smooth so the tile will always look nice. Also, will peel and stick tile adhere well to bare wood, and if not, what should I apply to the floor first? Thanks for your help.

I think a sub floor would be appropriate to even out and cover those cracks, otherwise the tile will sink into those areas and you will not get an even look. Anyway, the floor, before applying stick on tiles, should be free of just about everything and super clean or tiles won't stick. I don't know how you would accomplish that with old hardwood.

If you do not want to install a sub floor. You can get vey thin Luan board or like and glue down over the hardwood to make an even surface. You could also use caulk between the cracks if the rest of the floor is even. But if you have any uneveness the tiles may crack. WE also use extra glue to make sure the tiles stick well, just keep it away from the edges to keep from running between the tiles. My husband does a lot of rehabbing on the side and we have used this method on quite a few of older floors.