Totally gonna depend on your 5 year old...these work with mine, but rarely with his little sister!!
We have a list on the wall of different snack categories (dairy, veg, fruit, savoury and sweet) and the idea is to choose one snack from each throughout the day and each snack has to be accompanied by a drink. So we have babybels, cheese strings, yogurts, or a croissant, crisps, or veg sticks, bit of fruit, or sweets is stuff like dried fruit, chocolate bar. It varies depending on whats in the house but this lists them out. Helps encourage variety and limit the sugar.
Is it just looking at limiting sugar, which your title suggests? Or generally eating a varied diet?
At the table we can be quite playful, especially when it's a new meal and we're encountering resistance to trying it. I'll load the spoon up (not for every meal at all, it's just when we're being fun) and say 'now don't you touch that, I'm going to have it after I've had a drink', then lo and behold when I turn back the spoon/fork is empty. shocked face where did my food go?!
We talk about the texture and taste of things, 'which do you think will crunch louder, the pepper or the carrot?'
Mine is also very competitive. One day I realised he'd had something like 10 different fruits and veg across the previous day and pointed it out. Then since when we've been in a 'veg are the devil' stage I remind him of that day and he sets out to beat his own record. His current record is about 18 I think!
Otherwise hidden veg is great. In a 'cheese sauce' you can put things like cauliflower, sweetcorn, carrot etc blended up, or in a tomato base for pasta and pizza you can hide all sorts. Lasagne and sausage casseroles are great for sneaking in veg.