I randomly searched the word "胡思亂想", so many translations came out：

1. have a bee in one's head woolgather; Have a bee in one's bonnet

2. make blind and disorderly conjectures;

3. be lost in various fancies and conjectures;

4. be moved by confused, foolish reflections;

5. entertain all sorts of ideas; entertain foolish ideas;

6. go off into wild flights of fancy;

7. have a maggot in one's brain [head];

8. indulge in flights of fancy;

9. let one's mind wander; One's wits go a-wool-gathering.;

10. spoil one's head [confuse one's brain] by thinking nonsense;

11. think confusedly

12. muddle

13. let one's imagination [fancy] run wild;

14. random thoughts

Which one do you prefer? or you have any better idea?