I'm just wondering, apart from being able to build your own house and adopt children, does Hearthfire add any quests to the game? or anything else that might be interesting?

You just summed up the point of Hearthfire: building a house and starting a family.

It depends on what you mean by interesting. It doesn't add anything to the world, everything is related to your house and family. It adds new activities like baking, farming, fishing. You can hire a carriage driver and a bard. Sometimes your house gets attacked by bandits or monsters and you have to protect it - your spouse can even get kidnapped.

It's a small DLC, but it's fun.

Oh alright, well still sounds like an interesting dlc

You can check http://www.uesp.net/wiki/Skyrim:Hearthfire page on UESP for more details.

It sort of a hit or miss DLC. If your character is a family man, you'll enjoy it - but if all you want is another place to dump your loot and leave without even saying goodbye to you wife and kids, then maybe it's not for you. It's a good gold sink though. Make sure you stock up on iron ingots before you start furnishing your house!