re: Planning a first trip to Paris, is it bad to go solo?

Les Invalides: LINK
It's a big complex on the Rive Gauche (left bank) & it houses the Museum of the Army. If you're into suits of armor, military history, etc, it's a pretty cool place. IIRC, all/most of the descriptive & explanatory text is in French. So either start working on your DuoLingo French lessons or get a good pocket dictionary or translation app if you want to know exactly what you're viewing. IIRC, there is a simple museum cafe at the rear of the complex near Napoleon's tomb offering a daily hot lunch special and sandwiches, etc.

It's also within easy walking distance of the Musee Rodin, which has an indoor museum and a very nice outdoor sculpture garden full of roses. If the weather is nice, you can buy a ticket to just the garden portion and stroll/view/enjoy being outside.

Another Paris tip: while the metro is easy, quick, and goes all over town, don't be afraid to take the bus. The buses alllow you to see the scenery/streetscape, and you'l get a sense of distance and layout that you do not acquire when whizzing along underground in the metro.