Keep in mind: bones are not only made for standing upright, they also act as origins and insertion points for muscles and tendons.
Best approach: an exoskeleton with muscles attached at the inner side of the exoskeleton or at thorax pit like structures as seen in the opisthosoma of tarantulas.
Another approach: skin ossifications acting as endoskeleton. An example for skin ossifications can be seen in crocodiles (made of bone) or (made from calcite) in sea stars, sea urchins, and sea cucumbers.
A gas filled body is also possible but needs a huge amount of pressure...
Have you thought about a hydrostatic skeleton like the one seen in round worms like Ascaris megalocephala?
But: everything creating pressure from the inside makes your human-like creatures quite vulnerable. For example: piercing Ascaris with a needle leads to a worm-liquid fountain of 20-30 height. Things like stabbing your human-like alien would cause quite a fountain ;-) hurting the gas-balloon inside your alien would have weird effects, too. In both cases (liquid and gas filling), you need a really quick and advanced repair tool.