Learn English – word for “active activities?”

The term is Patient, which is formed from the same semi-deponent Latin verb (patior, patere, passus 'suffer, endure') as Passive. Most grammatical terminology comes from Latin, so if you want to make up a linguistic term, get a Latin dictionary.

In a prototypical English active transitive sentence,

• the grammatical relation Subject has a semantic role Agent (< ago, agere, egi, actus 'do'),
  while
• the grammatical relation Direct Object as the semantic role of Patient.

If you're committed to a single word, I would look at the reflexive verbs lull, quiet, or ease. Soothe would work.

The resulting sentence would be something like,

"I lull myself by the river."

"She quiets herself by the river."

But, if you want a single unreflexive verb, I would use unwind or relax (somewhat obvious).

"I unwind by the river."