Want to get fast XP: If you are level 15 or up and you have apple trees make sure you pick the apples you will get 7 XP per apple. Try and find apple advertisements in the paper, then visit the people that have apple advertisements then search their farm for shaking trees with exclamation marks on a sign, click on the sign and you get an XP reward. Complete orders on the order board. Visit other people's farm and look at their dock; if it has a crate with an exclamation mark click it and take a look at the supplies you need to get an XP reward.
Cats and Dogs
You can buy the cats and dogs with vouchers and you gain vouchers from completing boat and truck orders or harvesting crops and treasure box. You can not trade them. When you have enough vouchers you can buy your cat or dog, but before you do this you must buy a cat or dog house. Your cats and dogs will need feeding, milk for cats and bacon for dogs. Every time you feed your pet you will get 30 xp. They only feed every few hours. Apart from the xp they are mainly a decorative purpose for your farm, or for some players, it is a achievement getting each individual pet.
Finding Missing Crops
If you have run out of a certain type of crop, go into your friends tab and select Greg. Enter his roadside shop and buy everything that is there. Sometimes there is even some tools there for a very low price. If Greg does not have what you want, visit other farms and go thought their Newspapers and buy what you need in bulk (like 10 wheat for $x).
Provided you have an available slot you can sell stuff at the roadside shop as often as you want. You can only get a free advertisement in the newspaper every 30 minutes.To read more information about Hay Day or other casual games, you can go to 9Game.