Budapest is one the places on my list to visit (hopefully soon).  Any suggestions on what to see and do?

If you like historic places I would recommend  St. Stephen's Basilica , up the river a little is Szentendre Centrum and Visegrád Castle Ruins. If you like historic cafes you will not want to miss going to Centrál Café (Budapest) and Tabáni Terasz (Budapest).

There are many other things to see and places to visit.

What kind of things are you interested in?

mostly architecture

Although these are physical location and structures, it is the history behind them, the role the played in Hungarian history, and the food and coffee they serve that make them worthwhile for visitors to check out.

There are also a lot of thermal baths that are fun to goto.