During the day I make the dc up a sport bottle of juice (dilute squash or diluted fruit juice usually, water occasionally but they aren't very good at drinking it in that case). Then they can keep on drinking throughout the day without the drink getting knocked over or flies getting in and I can top it up as required. Or if we are going out it is an easy thing to grab them and go.
I always have a pint glass of dilute elderflower juice on the go too and they know they can always drink from that too if they are really thirsty.
Lemonade is a very occasional treat - and if it's hot and we're in need of a real treat, then ice cream soda is the order of the day (mainly because they were a big deal treat for me when I was little and it's a nice little tradition to continue although I don't think the dc like them as much as I did. And in this miserable summer it hasn't yet been hot enough to have them other than that one good week - which we were all in bed with flu for so we didn't get around to even thinking about them).
If we are going on a long car journey then I tend to pack lots of cucumber and melon, wash it before I go but then cut it up whilst going along (obviously dh is driving, I'm not cutting and driving at the same time!), (I know, I am sad, I have a designated car cutting board/tray and knife that comer everywhere with us).
I also have a pack of tesco basic plastic disposable cups (or are they glasses if they have no handles on despite being white plastic?) Anyhow, the dc get one cup/glass each and I keep refilling with melon or cucumber as required. They keep topped up and refreshed without needing lots of juice to be swilling around in the car. The cups are perfect for them to hold in the car (either in their hand or in the cupholders between seats or one of the child seats has a cupholder) - much less likely to get tipped over or for things to slide off than a plate, and very cheap so I just throw them away at the end of the journey if they are damaged.
DS2 loves coconut water (so do I) which is sometimes available from the world food section of big supermarkets - used to be about 30p a tin. But then apparently there was a big shortage of coconuts - reported in terms of coconut cream shortages on the news - and ever since it has been much more difficult to get hold of - plus it is now about £1 a tin so significantly more expensive. But lovely and refreshing, especially when fresh. I saw one of the green coconuts for sale in the local supermarkets recently and got it, thinking it would be lovely to have the fresh juice. I barely made a dent in it with my big chopping cleaver so asked dh if he could drill holes into it to let the juice out for me. I also made the mistake of reminiscing about the man at the market where we used to live who opened them up very easily with his machete...
Unfortunately dh took this as a personal challenge and reappeared with his axe. Let's just say that the score ended up something along the lines of coconut 37-axe 1-chopping board retired due to fatal injury 
And he didn't bloomin' listen to me before doing the sodding chopping because when he finally did chop a chunk out of the coconut the juice was all over the floor in a moment. arrrghh. and whilst the coconut jelly was very nice on the inside, it wasn't worth the loss of the juice, the loss of the chopping board, or the high risk attack on the kitchen floor tiles, not to mention the worry that dh was going to chop his hand before he chopped the coconut!
Neither dc will drink milk - dc2 was bf and I could never convert him. dc1 drank it at bedtime from a bottle but as soon as we refused to let him have it from a bottle he refused to drink it as he said it tasted funny. Tried all sorts but never managed to convince either of them.