Lindsay Lohan -- EuroTrip Post Rehab

Lindsay Lohan has booked a one-way ticket to Europe ... set to depart as soon as she finishes rehab ... because everyone knows an overseas vacation is the best way to keep your nose clean.

Sources close to LiLo tell TMZ ... after her 90-day-court-ordered rehab finishes up she'll head to London for some R&R ... as the first stop on a European adventure.

We're told a gal pal of LiLo offered to take her as a gift for a "job well done in rehab". And since LiLo has some time before her OWN docu-series begins filming she jumped at the opportunity.

Our sources say she booked a one-way ticket and has no return date set (yet), but hopefully it's before she's scheduled to film.

Lindsay believes the trip will give her some privacy during her post-rehab days.

Plus, it's Europe ... what could go wrong?