I agree on playing with plants, but building stuff is pretty awesome.
There's a patio behind my house, well, two of them, linked by a retaining wall and limestone stairs, plus the two mega retaining walls for the garage. Everything was done by us... My father, who was the only one with the skills to build the wall, and strength to lift the blocks; my mother, who largely helped him arrange the patio blocks; me, digging and carrying patio blocks; and my sister, picking out the patio blocks and pegging the walls. Four of us, and it took the entire summer, or maybe two summers. I don't even know when/how my parents worked their normal jobs during that. (I was 9 or 10 at the time.)
The patio itself (twice the size of yours) was a matter of a few weeks, I'd guess mostly on weekends. It's not easy, and this one was made better by having two children to fetch blocks and bring them over, rather than standing up and crouching down over and over again. Tools... chalk line, level, tamper, wet saw, mallet, pry bar, trowel, spade, shovel, wheelbarrow. Not all that much actually.