I like a bit of comedy when I visit an art museum. Such as this picture, one of four frescoes at the National Gallery in London, Domenichino's "Apollo Killing the Cyclops," with a dwarf in the corner peaking out from behind a painted curtain, as if the fresco were a painting of a tapestry:

Or Jan van Huysum's "Flowers in a Terracotta Vase," with its wonderful detail of a fly so realistic that one could think it was a real fly that had landed on the painting:

It's down on the pedestal of the vase, slightly left of center. Here's a closeup of it: