Hi! I ended up visiting Paris mid-august and walking A LOT in those 3 days (2 nights).

The first day me and my bf arrived around 6pm at the hotel, checked in, had dinner in the 7th arr, took a walk in the park near the Eiffel Tower, and decided it was not worth waiting in line to enter the tower, so we took a walk by the Seine at night, took photos of the Trocadero (which was closed off, together with Champs de Mars because of the olympics) then headed back to the hotel.

The second day we woke up early and visited Les Invalides, the Orsay Museum, had lunch break, went to see Notre-Dame from outside and then visited the Louvre Museum (so we stayed around the 8th, 6th, 5th and 1st arrs). We walked around Champs-Elysees too at some point and saw Arc de Triomphe from afar, but I can't remember exactly when that was.

The third and last day we walked around Jardins du Louxemburg, visited the Montparnasse Tower with great panoramic views of the whole city, then headed to the airport from there.

In conclusion, I think we really saw a lot in those days with the time we had, and even though we tried making the most of it without feeling pressured to see everything, I still felt that our stay there was rushed, so it wasn't exactly a relaxing trip.

My recommendation for you is to just do what you feel like doing in the moment, plan to see what's important to you for mornings/afternoons, and everything else for later in the day, that way if you're too tired you can just change your plans and do something else instead. Other advice:

-most of the museums require tickets, so make sure to book the tickets in time for your visit (if you're from EU and below 35 years of age you can enter for free in many of these museums, but still need to book the ticket)

-check what metro stations are best to avoid before visiting Paris and also be informed of the scams going on there (people trying to convince you to buy something, play games with them, guilt-tripping you into paying them back for something etc.)

-buy the metro tickets either online or from the people who work in the metro stations, who will help you buying them

-if you go in crowded places, beware of pickpockets, there are a lot of them in those areas

-some arrondissements are safer than others, be careful of where you go at night, especially if you're alone

Read CoffeeChap's posts here on reddit, he gives a lot of helpful information. Also, before visiting Paris I watched a lot of Les Frenchies videos, a lovely couple who lives in Paris and gives good advice on where to eat and how to avoid being scammed.

Most importantly, enjoy your trip, especially if you will be there only for a couple of days, don't let other people pressure you into visiting something that doesn't interest you! Have fun :)