Looks like a nice start.
If your going to be modeling an entire theme park, then I would recommend using Mental Rays Approximation Editor to create Rendered Subdivisions. This way you can have your Lo-Polys base meshes in scene whilst operating, yet have them Render smooth in Mental Ray. This will save on memory, and make things allot easier to operate.
Also use Display layers for your objects, or groups of objects. This way you can have you entire theme park in one scene, but operate on certain areas individually by hiding the others on they're separate layers. This will also help save on memory Lag.
Thats pretty much all I have for advice, though you may have known about these things before, but if not.... there great tools to use.
As for things to add to the Theme park. You could have a Ghost Train, Roller coaster, Petting zoo, also you might like to have a little 'Cabinet of curiosities' there always a good touch
Anyway, what you've got so far is looking real good!
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