3 Answers

Check the topics here...https://www.google.co.uk/search?q=presentation%2C+about+why+someone+should+buy+a+certain+product+instead+of+an+ordinary+one%2C&rlz=1C1CHIK_enGB517GB517&oq=presentation%2C+about+why+someone+should+buy+a+certain+product+instead+of+an+ordinary+one%2C&aqs=chrome..69i57.8340j0j4&sourceid=chrome&es_sm=122&ie=UTF-8

Let them sink in, and LEARN!

How about which type of backpack a kid should use, some make great stress to a growing childs body, others not so much... 

country bumpkin

When my son was in the 4th grade I took his backpack to the post office to have it weighed because it was so heavy his back was hurting. His backpack weighed 19 1/2 lbs. That's a lot of weight to have to carry for one of the smallest kids in class.

Shoes - high heels vs. low heels; different types of sport shoes
Jeans - there are so many different brands and styles. 
Socks - ankle length, calf length, knee length, color vs black or white
Cereal - pick a couple and compare the nutrition and the flavor
Soda - do you have a preference?  WHY? 
FAST FOOD restaurants - you must have a favorite place...food, price, hours, location??

                                            

My advice would be to think of things you prefer to buy and go with something you know! 
Make a list of pros and cons for each item.  Take a survey and see what 20 different people prefer.  Boys and Girls.  Ask THEM why they prefer one over the other.  
It could and should be a very fun project.  Good luck!!