Drøbak's old centre is small, pretty, and unforced. White wooden houses climb above the harbour, galleries and cafes sit along the lanes, and in summer the whole place has the relaxed confidence of a town that knows Oslo is close but not too close.
Do not rush it. Walk from the square down to the water, look across to Oscarsborg, and take the side streets rather than only the main harbour line. It is the atmosphere, more than any single building, that makes Drøbak worth the detour.