[RickOShay]

Originally Posted by Sashimister

This isn't nearly as easy as other Jaapnese learners might tell you.

なんのてんぷらですか can only mean one thing and it's easy to use.

Tenpura can be made with many different food items, all types of seafood, and vegetables and such. Suppose there's a piece of tenpura in front of you. You know it's tempura from its look but you don't know what's inside the batter. This is when you say なんのてんぷらですか.

どんなてんぷらですか , trust me, isn't too easy to use.

Suppose someone is telling you about the tenpura he ate in Tazewell, Tennessee. The town has no Japanese population and it has little connection to anything Japanese whatsoever. You know a lot about tempura and Japaense culture in general. You want to know how the Tazewell tenpura looked and tasted even though you already know you're going to hear a horror story. This is when you say どんなてんぷらですか.
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Another meaning of どんなてんぷらですか is the same as なんのてんぷらですか. However, if the tenpura is right in front of you, you don't say どんな. You say なんの. This is super-important.

You use どんな when you ask someone who ate tenpura not too long ago about what varieties he had.

This is an excellent explaination. But to the OP, do not get too caught up trying to remember every little difference. At some point it is probably more effective just to listen to how native speakers talk, and repeat it just like they say it, and when they use it.