The London Tour has begun in Mario Kart Tour, which means there are new challenges to complete. Completing these challenges earns you Grand Stars, which you need a lot of to gain access to the later Cups.
You can find Festive Trees dotted on racetracks throughout the Tour. London Loop, Mario Circuit 2, and Daisy Hills in the Daisy Cup all have festive trees. To do this challenge, you want to at least race at 100CC. If you are in first place, you are more likely to get items like Bananas, which you can’t use to hit the festive trees.
Being in the middle of the pack increases your likelihood of receiving items like Green Shells and Bob-ombs, which you throw at the trees as you pass. Crashing into the trees won’t work, you need to hit them with an item from an item box.
This challenge is merely grinding and saving your items to hit the trees instead of other drivers. You also need a little bit of luck on your side because the item boxes might drop unhelpful items.
There also appears to be a bug with this challenge, where it could complete early for you. I only hit two festive trees with an item, but the challenge completed for me after the race. I can’t say that this will always happen, but it’s better than making the task more complicated.