Is this your first dog ?
If so, and you have children, it would be best to stay away from any breeds that have guarding as an inherent trait. We all know fabulous rottweilers, dobermans, german shepherds etc but the reality is that the risk level is higher and they require experienced owners and, ideally, a defined role for them that suits their natural instincts.
There is a reason why labradors and border terriers are frequently recommended as good, family dogs for those without experience.
A working line lab would give you the activity levels you need - if it has been bred well and you do the research.
A showline lab would be a good introduction into labs but possibly less likely to be able to do agility.
A border terrier could be a managable, if slightly independent, happy addition to your life. A smaller dog might be an easier introduction into dog ownership and generally they'd cope with the family, the walks and the agility.
Vizlas have been recommended - frankly they can be amazing or bat shit crazy and you wouldn't know which you're getting until you're in too deep
Golden retrievers - lovely breed but again there's more unpredictability
The larger the dog the less likely they are to be able to do agility but there's always alternatives like gun dog training etc. A large dog will be a large puppy so that's something to factor in for the sake of your children.
Whatever breed you decide to get, do the research and do the training but don't set yourself up for failure by getting an intense breed as your first and prepare youself by researching both the best and worst traits of each breed you're considering.