Amanda Bynes' Bikini Body -- I've Done What the Dr. Ordered

Amanda Bynes got a side benefit from intensive therapy for schizophrenia and bipolar disorder ... a bangin' body.

Sources familiar with Amanda's treatment tell us ... doctors told her daily exercise was an important part of regaining balance and calm, and it's very clear Amanda has been a good patient.

So this week she's been chillin' in Mexico for Spring Break -- doing it up CABO STYLE.

The trip was a present from her parents, who chaperoned her ... meaning no beer, body shots or wet T-shirt contests.

She's on break from her fashion design school, where she's done incredibly well.

Amanda -- who turned 28 Thursday -- clearly learned a thing or 2 in school, 'cause she looks great in a green bikini!