The "Smashable" Brick Challenges are the orange ones in the challenges menu. They require you to smash through certain LEGO objects across the map. Most are pretty self-explanatory. Some require you to use a specific vehicle. I highly suggest you check the auction house first when buying cars so you can get them for cheaper. I've listed all the challenges below with their description + some locations were they can be done.
Smell, But Don’t Stop – 500 flowers in 30s – Fields by the festival site
Too Cool for Driving School – 20 traffic cones in 45s – Cones are lining the road in front of your house
Lacking in Basic Tract – Ram into a parked tractor – Go left from your house, one is located on the right side along the main road.
I’ll Take One to Go – Coffee cart – Some are located by the beach near the Mainland fast travel, between the main road and the umbrellas.
Rubbish Driving – 10 rubbish bins in 60s – They’re green, found in the city.
Running the Red – 10 traffic lights in 30s – Found in the city.
It Was the Tide? – 10 sandcastles in ’67 Stingray – By the pirate ships on the beach.
Bye, Plane! – Ram into a biplane – Some in the airport.
Change of Itinerary – Ram into passenger plane – Airport.
Truly Extinct – 30 dinosaur bones in offroad buggy – In the Jurassic Graveyard, just hit the small ones lying on the ground.
But Do You Have A Flag? – 10 beach flags in Rally Fighter – By the umbrella chairs on the beach.
Action Movie Mayhem – Ram into helicopter – Airport.
Watch Your Step – 50 LEGO brick piles in 30s – Tons of piles in the stunt park.
Zero Monkeys Found – 50 barrels in 60s – By the pirate ships on the beach.
Overcaffeinated – 3 coffee carts in 45s – 3 are located by the beach near the Mainland fast travel. They are between the main road and the umbrellas.
Gold Neighbors – 200 fences in an extreme offroad vehicle – They are along most roads.
Bad Luck – 7 ladders – They are brown, in the city.
Framed – 10 bikes – Most are in the city.
Soak It All In – 10 umbrella stands in Mazdaspeed MX-5 in 20s – Beach near Mainland fast travel.
Have A Seat Over There – 30 chairs in 20s – Beach where all the umbrellas are or in the city.
Smashing – 500 bonus cubes – Biggest cluster is on the airport runway.
Don’t Tell Me What to Do – 100 signs.
Table Flipping – 100 tables – In the city. They’re small and circular, not the picnic table ones.
You’re Not the Boss of Me – 500 road barriers - Along the racetrack.
Let me know if you need a better explanation for any of these challenges and I can add more to it.