Welcome to Paris, Steve.
I've been coming to Paris for 35 years and have lived here for half a dozen.
A couple of thoughts, therefore.
You haven't chosen the best time. Paris is full of tourists and there will be long queues for everything. Many Parisians are on holiday and many shops and restaurants are closed. Those that are open often cater very much to tourists. Under no circumstances attempt to eat drink or shop on the Champs Élysées: it's the biggest tourist trap in the universe and you will pay a fortune. For shopping, try the Bon Marché on the Left Bank or the Boutique de l'Hotêl de Ville (BHV) near Chatelet.
Paris is incredibly dense - a small city which has been developing for a thousand years. You can spend an hour in one street and an afternoon in one small area.
Don't be surprised by beggars and poverty. Successive governments have pushed the economy in a much more Anglo-Saxon direction, and extremes of wealth and poverty are much more common than they were. Most beggars are illegal immigrants from Africa and eastern Europe, but increasingly they are middle-class people thrown out of work. Give them something.
You can divide tourism in Paris into buildings, museums and places.
Buildings (not necessarily to enter) include obvious things like the Eiffel Tower, the Invalides and Notre Dame, but also the Institut du Monde Arabe, the Panthéon and the Opéra.
For museums, the Musée d'Orsay and the Louvre are worth seeing just for the buildings, but there are so many museums that it's better to decide what interests you first, and then visit the museum. The Musée du moyen age, in the Latin Quarter is outstanding, and is in housed in a medieval nunnery. The Musée du Quai Branly is also outstanding as a collection of so-called primitive art.
Places are what Paris is really about. Squares - Place de la Sorbonne, Place des Voges where you can sit in cafés. Walking down by the river. The Ile St-Louis beyond Notre Dame. You can spend an afternoon in areas like the Marais (old Jewish quarter, now very trendy), the area between Sèvres-Babylone and the river, taking in the Rue du Bac, the Odéon, St Germain des Prés up to the Boulevard St Michel. The other side of the Boulevard is the Latin Quarter with its university, bookshops and medieval streets. And don't forget the gardens: the Tuileries, the Luxembourg Gardens (my favourite) the Jardin des Plantes and the gardens of the Palais Royal.
And do as the French do: sit in a café and watch the world watching you.