First, you need to complete this week's assessment task which can be found here:
Year 6 Assessment - Perimeter and Area
For the rest of today's lesson, you have the choice of four challenges. Challenge 1 is the easiest whilst Challenge 4 is the hardest. You can complete as many as you wish (though don't spend more than an hour in total). Each challenge has an explanation - make sure you listen to them so you understand what you should be doing. The answers for Challenges 3 and 4 can be found by following the 'solutions' link on their explanation pages.