I know you asked about soloing, but most of this stuff will be much better for you in groups...At your level for the best xp I think the heroic mobs will give you what you're looking for.
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Grab one person around your level (tank + healer is great duo of course) and take a left when you zone into Thundering Steppes from Antonica. Head to the beach and locate the stone there out in the middle of the sand that glows when you mouse over it... you can zone in there for some heroic mob pirates that give you purty good xp ;)
Over by RoV zone in, there are a series of craters. If you take the main road from where you zone in Ant>Thundering Steppes head to the area between Q Towers 3 and 4.. the road will curve around to right... don't go with that big curve, instead head kinda straight and to the left side of the big mountain (you'll see some tall rock formations along where you head straight alongside this left side of the mountain). If you run directly along this mountain wall on left side, it will lead you to a particular crater where I gained lots of xp around lvl 28 with a group, pulling heroic skellies to the top of the crater there.
And let's not forget Firerock Giants ;)
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Ruins of Varsoon itself, which will require a group, yields good xp as would Stormhold.
Get up a group for Stormhold (in Antonica) and journey into the deeper areas below the library past the old jail cells. I believe, btw, there is a hidden entrance down there that you can quest for. All over Stormhold however you should be able to solo things as well.
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Treants may be a good idea for you to get in groups in Nektulos. Try to have SoW and be prepared to run! LOL
I'm just into lvl 30 and beginning to explore Enchanted Lands which would be awesome xp for you but again, at 28 you'll need a group there. The Zek and EL access quests are good xp within themselves (have a good group for the boatrides my friend)
Just a few things that popped into my head ;) Good luck lvling!