The best way to visit Paris is on foot with occasional use of the metro, city bus or taxi to take us from
point A to point B (and it will cost you less than if we use my car). But sometimes the best way from point A to point B is not necessarily the shortest one. Perhaps there's a less crowded route, or one where the sidewalks are wider (there's that social distancing again!), or maybe there's a building or some street art nearby that would be a shame to miss. A local guide is a better choice than Google Directions!
To get around on the metro (not recommended right now), you could buy metro passes but you would need to use the metro seven or eight times a day to make it worthwhile. I recommend buying a booklet of ten tickets (a "carnet", pronounced CAR-NAY) that you can use as and when you need them.
And after you've been with me for at least half a day, you'll know how to use the subway like a local!
Otherwise, a lot of you I know are attracted by the hop-on/hop-off buses. I find them a little bit expensive for what they are. In addition you would be dealing with an imposed itinerary which doesn't take you to places in the order you may want to see them. Admittedly, the upper open deck is great for taking pictures, weather permitting.