White and purple clover, if you can find it (generally if you see it, civilization is nearby). Pick the individual flowerlets and suck the sugar out of them. Just need a lot of it.

DO NOT use yellow clover, that isn't so good for humans.

But realistically, other than fruit and maple and birch sap during the sping, there isn't a huge amount of sugar in naturally occuring plants. Thats why tooth decay was relatively uncommon to most pre-technological peoples.