Just complete one challenge. If you do this after you have done career all levels are farmiliar. I did mine in the black garden with enough time to spare. There are 2 type of challenges. Time and water. Time being clean the location or item in given time for gold or Water for efficiency.
Tips i can give is use the soap nozzle + soap on items you struggle with. This should give you a good edge on the challenge to come.
I found these 5 the easiest to complete for me:
Locations:
Back Garden (1 try)
Vechicles
Van - Time challenge (2 trys)
Dirt bike - Time challenge (5 trys)
Golf cart - Time challenge (1 try)
Vintage car - Time Challenge (4 try)
If you feel like another one is easier. Pick that one. You have enough challenges to find 5 that you can do hopefully.