I would find out what hotel
ABD is doing it's pre-night at (assuming they are providing pre-nights). When I went, it was the Budapest Marriott Hotel. They not only did the pre-night there, but it was the gathering spot for transportation to the port. It's hard to know if things are running the same as they were pre-pandemic, but we met with the Adventure Guides in the morning at the Marriott, and chose our excursions, then they provided multiple bus runs to the ship It's very nicely located right on the Pest side of the Danube, and has wonderful views of the Palace on the Buda side of the river. It's walking distance to the Chain Bridge over to the Palace, which is not included on ABD, and could be a nice pre-day trip (there's also a funicular there to the top of the Palace hill). Personally, I did the pre-night, and was glad I did, but they can be pricey. I would ask ABD if you can use their transfers to the port if you don't book the pre-night with them.
If you are planning on getting yourself to the ship, you'd need to find out which port the ship will be at. There are 2 in Budapest, and one is near the Marriott (but was NOT the one the ship was boarding at!).
Budapest is a gorgeous, very walkable city. I highly recommend the Dohány Street Great Synagogue. It's the largest Synagogue in Europe, and is simply gorgeous, both inside and out. It's in a beautiful Moorish style. They do run wonderful tours, or you can get a pamphlet and walk it yourself.
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