We just had a summer (well 16C in central NSW LOL) Christmas weekend with 6 children 5,4.5y,3.5yo 2x 2.5yo & 9mo) for 2 days together (none of us live close by) & so had heaps of time together.
I managed to get some glow sticks (from carols) & they played with them for ages & bubbles they all loved them, the 5yo & 4yo were blowing them for the younger ones. My 4yo got the Hungry Hippo game for Christmas & that was played with for ages, as was my Sister's keyboard & they played statues (ie stopped when the music stopped) & did stickers to decorate picture frames.
From my experience of my eldests 3rd party, games are a bit hard with the age range & more "informal things".
Not sure on your water restrictions but water play is a good one if it is hot (your Melb aren't you... so could go either way) , sauce bottles filled with water to "paint" the concrete. 1-5 can be quite a hard range depending on the children, so I'd be inclined to have ideas & go with the flow, rather than anything too planned.