Seconding everyone’s suggestions so far!
I recommend Angelina for the most amazing hot chocolate you’ll ever have 😍😍 And if you’re looking for a reasonably priced traditional French lunch or dinner, try out Bouillon Chartier, a chain of restaurants in central Paris - you normally have to queue a little bit to get in, but it is so worth it.
Also, if you’re looking for somewhere to eat at lunchtime, loads of restaurants will have fixed-price menus and these are normally affordable and really nice. Paris obviously also has bakeries for ten a penny, which are an option for a quick breakfast or lunch. One of my favourites is Du pain et idées, in the 10th arrondissement, near the Canal St Martin. This is fairly central, and it’s nice to walk along the canal with a pastry in hand 😊
Someone mentioned Parc des Buttes-Chaumont, which is a lovely park/meadow with lots of wildflowers and wild areas to explore. I also recommend the Jardin du Luxembourg, which is a really nice park in central Paris.
If you’re looking for good views of the Eiffel Tower, you can get a really good one by the Sacre Coeur basilica in Montmartre, or if you go up to the upper floors of Galeries Lafayette department store on the Boulevard Haussmann.
Also recommend doing a boat tour on the Seine, especially if the weather is dry and pleasant. It’s a really good way of seeing Paris.
And if you like art and museums, I recommend the Picasso museum, the Musée d’Orsay (especially if you like post-Impressionist art - e.g. Van Gogh and Gauguin).