Itinerary Advice - May 19-24

My husband and I arrive May 19 from Zagreb via bus. Have booked the
Cortile Hotel on points (per Mr. E and RS guide advice).

Okay, I recommend location, not hotels. To recommend a hotel I would have to have stayed in it first. But if you are young (adults only pool bar on the roof) this one has a good reputation, and the location is not half bad.

Public transportation or Uber/Bolt from Népliget bus station to
Cortile?

Uber and Bolt in Budapest are both just metered yellow taxi cabs that charge the exact same as every other taxi company in Budapest…. So sure, if you have their apps loaded do it. More Bolts in town than Ubers so Bolt might be faster for a pickup. OR, the M3 runs from the bus station to within about a 4 minute walk to your hotel. Just use google maps to see the route.

Perhaps a Danube cruise may be a relaxing, low impact event for that
evening after a full day of travel (driving from Ljubljana to Zagreb -
bus from Zagreb to Budapest)?

Really pretty beautiful if you do the after sunset cruise.

May 20 we are headed west to the Pápateszér area with a guide to visit
my paternal great grandparents' villages.

Sounds rewarding.

We'll have three full days to explore the city. In every major city,
we hit as many musical performances as we can in the evenings. I have
already booked the Opera for our last evening 23 May.

Three full days will get you trough a lot of the city. Be sure to get a metro pass. Here is a pretty complete list of performance venues if you want to do more: https://community.ricksteves.com/travel-forum/hungary/concerts-in-budapest

And here is a list of restaurants that often (not always, you will need to confirm the date) have live music or entertainment.
DISTRICT II
Piros Pezsgő Piano Bar (M) https://www.facebook.com/pirospezsgo/
DISTRICT V
Rézkakas Bistro Terasz (M) https://www.rezkakasbistro.hu/
Café Liszt (M) http://cafeliszt.hu/en
Százéves Étterem (M) https://www.szazevesetterem.hu/
DISTRICT VI
Pörc&Prézli (M) https://www.porcesprezli.hu/en/
Callas Café (M) http://www.callascafe.hu/
VakVarjú (M) https://pest.vakvarju.com/
Sir Lancelot (S) https://sirlancelot.hu/
DISTRICT VII
Orfeum Club (S) https://orfeum.hu/
Spinoza Café & Restaurant (C) http://spinoza.hu
DISTRICT IX
Opus Jazz Club (C) https://opusjazzclub.hu/etlap
Jedermann (M) https://www.facebook.com/
DISTRICT XI
Szeged Vendéglő (M) https://www.szegedvendeglo.hu/
DISTRICT XIII
Budapest Jazz Club (C)
https://www.bjc.hu/home/

And if you get hungry for a burger, Zingburger is about as good as the get. Different locations throughout Budapest.

The sunset cruises seem the most popular, so that's something you should book in advance. You probably don't need to do it way ahead of time, but I'm not sure how soon is soon enough.

If you are looking for dinner and music, in addition to the opera, I loved the Opus Jazz Club (if they have something that sounds interesting) and Spinoza (as mentioned by Mr. Ë). The food was good and the performances provided variety (from the opera), both fairly small and intimate. Also check the Operette.

I thought the Legenda 1 hour cruise was done well. Board, find a seat, it includes a drink and someone comes to take orders and delivers it to you. Google Maps is accurate with the dock location, but give yourself a few extra minutes to locate how to get down to the river it in the dark. And truthfully I wouldn’t bother with it if not after dark. Maybe pair it with dinner at Dunacorso prior?

Edit: if you enjoy large events, check the schedule at the MVM Dome. I lucked into Il Volo one visit. Public transportation to and from was calm. Two days ago I arrived as the Katy Perry concert was ending. 🤣

Thanks so much to all!

Enjoy the opera! I saw Turandot when I was there several weeks ago and loved it. It was a beautiful production and the opera house is gorgeous. There are a lot of nice restaurants in that area as well, so you can eat before the 6:30pm start, or after. One place where I ate that is only a block away from the opera house is Két Szerecsen. It specializes in Hungarian traditional food, and my meal was delicious. They are open late, so you would have plenty of opportunity to go there after the opera is over.

For food, look at TTMs post https://community.ricksteves.com/travel-forum/hungary/restaurants-in-budapest-revisited.

I second the recommendation for the Legenda boat cruise. And definitely do the cruise when it is dark. It is amazing, especially seeing the illuminated Parliament building from the water!! One of the highlights of our trip. I checked to see what time sunset was when we were there in September, and then I planned the cruise for about an hour after sunset. I made our reservations about 3 days in advance. I kept checking weather reports to make sure I chose a night when rain was not predicted.

Sunset is 8:19 more or less on 19 May, so maybe the dinner boat at 7pm which will put you on the water after dark (its several hours); or the one hour cruise at 8:30pm