Vorosmarty ter is great, but think a bit outside of District V. District V is the river boat tourist district and its crowded and not very interesting and expensive ......... EXCEPT for the Danube Corso; especially in the evening. If you were to pick one point that was equal distant to almost everything a first time tourist would want to see in Budapest, that point would be Deak Ferenc ter. Deak Ferenc ter is in District V, right on the edge of it. So go across the street to Districts VI and VII and look there. But stay as close to Deak Ferenc ter as you can if you want to avoid metros and trams.
Personally, I think a short trip on the M1 tram is worth the effort to get a little more distance from the tourists and to get just a tad more into more traditional Budapest. The M1 is the oldest metro on the continent and not like any you have been on. Its just barely below the surface of Andrassy ut, the cars are small and the stations clean with antique charm (as charming as a metro can be i guess). Simple and not intimidating at all; and from the Opera for instance, the trip to Vorosmarty ter is only 5 minutes on the M1 or a 15 minute walk through some pretty nice architecture. Remember, what makes the city interesting is that it is a pretty well totally intact 1890's cityscape.
Oh, and there will be things going on all along the Danube Corso if it is like past years.