Interesting maths topics for year 5 and 6

I've been volunteering to teach maths to kids between 9 and 11 years old at our local primary school. These are the topics I've done.

Basic combinatorics

Probability

Eulerean path/cycle

Sides of a regular hexagon

Why are there no flying elephants

Moebius strip

Slide rule

Average speed: going uphill and downhill

Bike gear ratios

Measuring height using similar triangles

Conway's game of life

Why do astronauts float in space (not really maths)

GPS positioning basics

How do computers communicate in binary

Voronoi diagrams

Seven Bridges of Königsberg

Map projections

Synchronisation game

Everyone gets a piece of paper with a number on it, they pass the paper without communicating, in the end everyone has to say the same number. Extension: everybody has to say the same number, but the number must not be from the list.