In general, you can't do anything! Is the house uneven, then it is uneven, and needs fixing. You can always pad or build under the different furniture to some extent, but that mainly works for stationary furniture like a bed or desk.
When it comes to more mobile furniture like tables and/or chairs, these are not so easy to stabilise. Besides considering to build a new floor (possibly on top of the other floor), which doesn't seem like it is an viable option for you, mathematics has an interesting option, namely rotate the chair or table.
According to this article Strange but True: Turning a Wobbly Table Will Make It Steady, you'll be able to get all four legs in contact with the floor, aka being stable, with simple rotation of the table. Rotation, however, does not guarantee that the table/chair is level afterwards. So then it becomes a choice of the lesser evil: wobbly chair or uneven chair.
(added) On the feeling of balance
Note that the feeling of balance is not only physical. Objects can be level, but due to the surroundings they might feel unbalanced. In those cases you just have to adjust according to sense, or get used to the feeling.
When using a level, you use it on the object you want to make level, and then lift either end of the level to see where you need to compensate. Say that you need to lift the left edge of it, then you need to put more padding under the left edge of the object as well.