I love 'Easter eggs' placed in the movies. Easter eggs are the references or messages kept in a movie about another movie. I'll share some of the interesting things.

Source: Quora & HuffPost

1. Star Wars Episode 1 (1999) - you can find the alien character from E.T. the Extra-Terrestrial(1982) in the council meeting.
2. I am Legend (2007) - You can find Batman Vs Superman(2016) logo in one of the scenes
3. Monster Inc.(2001) - You can see the child presenting Nemo toy to Sully - Finding Nemo (2003)
4. Finding Nemo (2003) - you can find Buzz the Lightyear toy from the movie Toy Story(1995)
5. Indiana Jones and the raiders of the lost ark (1981) - you can find C3PO and R2D2 from Star Wars series
6. Frozen (2013) - you can find Rapunzel and Flynn from the movie Tangled (2010) in one of the scenes.
7. The Little Mermaid (1989) - You can find Mickey Mouse, Donald & Goofy in one of the scenes.
8. In the movie Fight Club (1999) - All the scenes has a Starbucks coffee cup somewhere in the screen.
9. Toy Story - Mickey Mouse picture can be found in the Andy's wall clock

Hope you find this interesting.