A combination of the volitional form of a verb and 〜と思います.
- Literal: I think I shall go to the park today.
- Natural: I (have just decided I) will go to the park today.
The present tense comes with the nuance that the speaker has just made a decision.
The sentence can be modified to the present progressive to indicate that the decision is not new.
- Literal: I am thinking that I shall go to the park today.
- Natural: I (decided earlier that I) will go to the park today.