1 -Return to the same place to do something new. (return to do)

2 -Go back to doing (not necessarily to the same place), or continuing to do something already begun. (return to doing, which is different from return to do, which is used also, wrongly in my opinon, in the sense of return to doing)

When you return to do something you have set a goal to actually do something after your return.

When you return to doing something you go back to the action of doing something, either deliberately or not (but it doesn’t really matter).

If you want the gramamtical answer why: two verbs in the same expression/action necessitates that the first be conjugated (I am, you are, he is, we are, they are) and the second left in the infinitive form (unconjugated. ie: [to] be, [to] eat, [to] run)

Thats the correct one.