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