While I "command" my sims with mouse clicks, most of "UI" stuff is done with the keyboard.
C - screenshot
P or 0 on numpad for pause, 1/2/3 on numpad for speed control.
F1 goes to live mode from build/buy, F2 does the opposite.
Page Up/Down - goes up/down a floor. Home/End - changes how you see walls (I was shocked when, in the terrain tools video, the guys actually clicked that somewhere in the UI where I'd not even look for it)
Shift-Tab for first-person mode (for some fancy screenshots)
Those are what I normally use.