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?