What were chosen as the demonstration sports at the 1956 Melbourne Summer Olympics?

Fitting for the land of Oz, the first demonstration game was Australian Rules football!

In front of a crowd of as large as 30,000 spectators, the Victorian Amateur Football Association (VAFA) caused an upset victory by 26 points over a combined team featuring players from both the Victorian Football League (VFL) and Victorian Football Association (VFA).

https://en.wikipedia.org/wiki/Australian_football_at_the_1956_Summer_Olympics

The second sport was baseball. The game was played between the US and Australia with 86,000 people watching in the stands making it possibly the largest baseball audience at the time of play. The US won handily 11 to 5 in 6 innings of play. The game had to be called early to allow the track and field and marathon to be completed.

https://en.wikipedia.org/wiki/Baseball_at_the_1956_Summer_Olympics

The game was played on 1 December 1956 from 12:30 p.m. at the Melbourne Cricket Ground. Due to the field being set up between the running tracks, right field was only 225 feet (69 m), so special ground rules were put in place, stating that a ball hit over the running track on the full will be declared a home run, where one that bounces (or rolls) on or over the track shall be declared a ground rule double. This rule was put in place to stop baseball cleats damaging the track for the events after the baseball.