You ask:
- It's possible to bookmark lines on a file without the need to name them and without the need to press RET?
- It's possible to have several unamed bookmarks on a single file?
- It's possible to cicle between them, also without the need to press RET?
Yes, yes, and yes. And much more --- using Bookmark+.
For the first two questions, see the doc section about autonamed bookmarks. For the third question, see the section about cycling among a set of bookmarks (e.g., the autonamed bookmarks in the current buffer).