Before we became more "vested" in the city we stayed at the K&K a number of times and it is still among the first places I recommend because its clean, well managed and in an interesting location. I have broken down over the years and have relented to the opinion that those with really short stays might be better suited to the Marriott on the Danube embankment. One night of the views of all the building lit up and the immediate convenience of restaurats and shopping (albeit tourist fare) might be worth the tradeoffs. BUT, you guys are coming on a cruise so maybe you have had all the river views you need for a life time. In which case back to the K&K for moderately priced accommodations or the Le Meridien for a more upscale accommodation. The K&K is in an interesting area and adjacent to other interesting areas while the Le Meridien is still a little in tourist central but being on the edge of the area not far from some interesting areas.
One full day Budapest
From the Hotel (either) up the M1 metro line to the Széchenyi fürdő stop (next to the last stop). Here is the city park the Széchenyi
Bath House (by about twelve for a couple tickets at the first station. you will need them).
Walk to Hero's Square
Walk one two stops beyond Hero's square (Kodály körönd station) for the architecture and the beautiful boulevard (the Champs-Élysées of Central Europe).
Board the metro to the Opera stop (two stops)
Walk from the Opera to Kiraly utca,
then down Kiraly utca to the Gozsdu Courtyard (at number 13 Kiraly utca). This will take you down some really nice typical streets and through some interesting old courtyards.
ON the corner as you enter the courtyard passage is Frappáns restaurant which is very good with some Hungarian specialties.
Into the Gozsdu Courtyard and turn right at the first large passageway.
Continue on and turn left on Rumbach utca and visit the Rumbach Synagogue.
After the Rumbach Synagogue continue on to the Dohany utca Synagogue (aka Great Synagogue).
Take the tour.
Stand in front of the Synagogue and look for a passing yellow tram.
Go the nearest tram stop (about a block south of the synagogue)
Take the southbound tram two stops, get off before it crosses the Danube.
You are in front of the Market Hall. (Watch the time as the hours here are: Monday: 06.00-17.00, Tuesday-Friday: 06.00-18.00, Saturday: 06.00-15.00, Sunday & Public Holidays: closed.)
After an hour in the market hall (be sure to see the basement behind the grocery store -- follow your nose) walk across the street. Now you are on the famous Vaci utca shopping street $$$$$ and tourists galore. Lets skip it.
Walk towards the river and down the stairs. Get on the tram under the bridge that moves from left to right. This is a magnificent ride.
Get off behind the Parliament Building
Look around as time permits.
You should be able to see the dome of the Basilica to the south east.
Walk towards the Basilica. There is good shopping for souvenirs etc along the mall in front of the Basilica. My favorite are very near the basilica on the right as you face the basilica and a music themed shop about half way down the mall.
Its late, go to your hotel.
But as you walk away take note of a restaurant across the street from the back of the Basilica with the name Lugas. That's where you will come back for dinner. My real first choice but a little further from the Le Meridian but very convenient to the K&K is http://maceszbistro.hu/
Oh, what about Buda? Stay another night. But after dinner you can walk to the embankment and look up at the lights of the castle. Very beautiful.
Might have more ideas if you have dates.