Well, there's been some great advice here and everyone's said pretty much everything I was going to say. I've had my Benjamin on a lead a few times, but only in the garden and I got an extendable dog lead to attach to it.
He hated having it put on then sat in a mood with me until I moved him and took him outside. I've not used a harness for ages and I don't intend to use them again, I've thrown them out.
Rabbits can escape from them, bolt and hurt themselves and can cause injuries to themselves whilst in them. They can't hid away whilst on a lead and feel controlled.
You also can't walk a rabbit, you can't lead them along. They have to go their own way and they walk you. Parks are definitely a no go area. Dogs, diseases, poisonous plants, other animal excretion, birds and the rabbit will feel extremely scared in a new environment as open as a park with different sounds and lots of people and other animals.
If you have a secure garden, with no gaps for the rabbit to escape then you can let your rabbit roam the garden (you still need to watch at all times as rabbits can quickly dig, chew their way out of places.. there's also the risk of large birds, cats or foxes etc).
I think the best option for taking your rabbits outside is a secure, large run. But still watch, as they could dig their way out.
Please, do not put rabbits anywhere near ferrets.