While studying this morning, I became interested in how to say "When you were a child, did you read books for fun?" in Japanese.
In this case, would I use ため?
Or would it be something like this: スパに行く食べ物を買う…。
I guess I'm wondering how to state the purpose of doing something, how to use ため, and also what are the differences in form are.
To say "As a child, Did you/I read books for fun" I would use
子供の時、趣味で読書をしていましたか？
〜で is simpler and the sentence flows better than 〜のために, which can be used to mean the same thing. However, when I say
子供の時、趣味のために読書をしていましたか？
actually sounds a bit awkward, since 〜のために (or ため in general) literally translates to "for" but it has a tiny bit of connotation to indicate whatever is done is a requirement, or something purposefully done. The reason why the specific example above sound a bit awkward to me is that your hobby is not necessarily something you have to do; you naturally do it out of enjoyment. So using 〜のため to indicate your purpose, I think, has an effect of emphasizing your will to do that thing.
If instead the sentence is
昨日、仕事のために読書をしていましたか？
then the awkwardness goes away, since the reading was a requirement for the job, something needed to be done by will.
In this case you CAN use ため depending on the grammar that you use with it. Specifically you could say 楽しみのため, or "for the purpose of fun," or you could say 遊びで, meaning just to have fun (rather than out of obligation). So a full sentence would be like 子供の時楽しみのために本を読みましたか？
As for the second example you gave, スパに行く食べ物を買う means that you will buy food that goes to the spa. スーパーに行って食べ物を買う, using the te form, will express the idea of going to the store and buying food, but NOT going to the store for the purpose of buying food. In that case you would also use ために, as in 食べ物を買うためにスーパーに行った。
