I would take a taxi straight to Ile St Louis (assuming good weather). Get him to drop you at the bridge connecting the two islands. There is often an accordianist on the bridge playing French "classics". And there's usually a party boat tied up at the nearby quai with music and dancing.

Walk around the perimeter of the island watching the river traffic and check out the stunning architecture on the island as well as the Right and Left Banks. Stroll along the main street with great shops.

Stop for a kir royale at Le Cafe Ile St Louis---hopefully you can get a sidewalk table. If you're hungry, there's a good creperie close to top of the main street near the bridge. OR...Berthilon Ice Cream is right down the street, famous for its unusual flavors.

I think you could get most of that in with just 3 hours and you will have had a tiny taste of this very walkable city. If you can't find a taxi on the island, you might have to walk over to the Right Bank. Allow enough time for that.

<Bendingo made an itinerary for you; however, I think that it is too long for 3 hours.>

My caveat is that I'm 6'2" with long legs, and I don't feel that I've walked a city properly if the soles of my feet don't feel bruised at the end of a day! :-)

PS

It is Place Concorde.

No one in Paris says Place de la Concorde.

And Bertillion is crap. It isn't the same as when I was a lad.

Go to Amorino instead if you think you must have a gelato.

Thin