The only things I can think of are dark chocolate and honey.
I don’t think unadulterated vegetables will ever be as sweet as natural fruit, but carrots, corn, butternut squash and sweet potatoes all have a high sugar content. Roasting them ups their sugar flavor. I recently learned slowly bringing sweet potatoes up to temperature converts more of the starch to sugar. I now start my roasted sweet potatoes in a cold oven rather than a preheated oven to encourage that transformation. It really makes a diffrence.
I am unsure of the nutritional value, but I find herbal teas like rooibos and peppermint sweetish.
Technically fruits but can be treated as vegetables and very nutritious, all the squashes - Butternut, Acorn, Pumpkin, Kabocha are my favorites. Also green peas, blackstrap molasses, Sweet Potatoes, beets, carrots, parsnips, Raw Honey (depends on the source, not very nutritious but has some natural enzymes and nitric oxide), red onions.
In what way is honey nutritious? It’s practically pure sugar. And really dark chocolate (80% and above) is not sweet, as the cocoa taste start to take over.
I’m not answering the question I know, so it might not be helpful, I just wanted to point out that anything sweet tastes the way it does because it contains sugar (or artificial sweeteners).
But here’s a tip: to bring out sweetness in many vegetables, you can try to boil or steam them. They’ll taste better, are easier to chew and digest, and you still get nutrients.
That’s all there is. Sugar isn’t healthy for you in large amounts. There is no cheat way to finesse eating whatever you want whenever you want while staying healthy. Eat in moderation, exercise, and you will be fine.