Well, solo mobs are hardly ever worth much xp. Quests can give a bit xp, but most over lvl 20 tend to require grouping (I'm not taking down giants on my own, even at 23, lol).
Still, Thundering Steppes has good solo material, better than those lizards, and several of them are needed for questing too. Try the antilopes (quest for them in Baubleshire), the beetles (both beetles and antilopes are quest mobs for trapper Borgus, although reward is low), the snakes, the crabs at the docks. (guild writ quest, if you are guilded that is)
Stormhold isn't all bad, but not that many solo mobs to fight there, some group mobs are green with arrows up and are doable. There are quests there that are quite good at lvl 21, the 50 leeches quest gives a very good reward, although the xp to be got from killing the mobs would be nothing at lvl 21. (some life leeches will be green though) Several quests involve Stormhold, starting in Antonica, unfortunately, for several that would require a group as getting to the atrium, library or underforge is impossible at lvl 21 or you would have to follow a group (or go just before a patch, apparently most mobs dissappear just before patching, they used to anyway)