For me, simplest quick excitement upgrade SWITCHES! they really add to the complexity of running trains, which adds to the interest. One or two straight tracks for a siding where extra rolling stock can be stored waiting for pick-up, or a curve or two for a passing siding. Two switches will connect the two loops of a figure eight which changes the train's direction of travel without increasing the size of the current layout.
Steam rules
"a country boy can survive!" (Bocephus, 1981)