My best pick for day activities will be “Montmartre”.
You can access through metro “Pigalle” and walk towards the Sacré Coeur cathedral which offer a great view of Paris, for free.
You can chill out in the park below it.
The inside of the cathedral is definitively worth it.
Then walk on the left side of the cathedral when facing it and explore Montmartre neighborhood. Stop for café, crêpes or drinks. If the crowd look French and reasonably young, the place should be ok.
Take your time to explore the less obvious streets. You many get lost, or find some very nice spots.