How do I import only some of my data from Snow Leopard to Mountain Lion?
This is definitely possible. For starters you may want to look at Migration Assistant, which will allow you to migrate only certain items.

If you want finer grained control, you can manually copy just about anything if you know where it lives. Most programs can be copied over without any trouble, and most apps have a folder in ~/Library/Application Support/ to store their settings and data.

Safari bookmarks can be transferred by either copying the ~/Library/Safari/Bookmarks.plist file, or using the Import/Export Bookmarks commands in the Safari File menu.

This blog post by Matt Gemmell has a good walkthrough of some items that may be useful to migrate manually.

If you need advice on transferring specific items, try searching for them directly on here or Google, or post a question specific to them if you can't find an answer already.