"algo" is much more usual than "alguna cosa" for at least the following reasons:
Therefore:
We do say: ¿Estás mirando algo en la tele?, but saying ¿Estás mirando alguna cosa en la tele? would be unnecessarily long as well as rather awkward.
The sentence ¿Es alguna cosa de comer? sounds a bit off, especially because of the preposition "de": "para comer" would be more idiomatic. Once again, ¿Es algo para comer? is both shorter and better. Similarly, I'd say Te compré algo para comer.
The question ¿Ha visto alguna cosa? sounds incomplete. It'd be more usual to find a relative clause or an adjectival structure after "cosa": ¿Ha visto alguna cosa (algo) en especial / alguna cosa (algo) que le llame la atención? I prefer "algo" in both cases, unless we need to emphasize the physical nature of the thing in question.
In the case of Hay algunas cosas que no te he contado, "algo" is not possible because it lacks the plural sense of "algunas cosas".