Opposite of disciplined with positive or neutral connotations

Perhaps free-spirited is what you want:

free spirit n. One who is not restrained, as by convention or obligation; a nonconformist.

AHDEL

However, I'd say that "the opposite of 'disciplined' ", if one can be agreed upon, is bound to have negative overtones because 'disciplined' has positive ones.

Spontaneous, perhaps. It's usually regarded as a positive word, but also implies a lack of discipline.

As hinted at in Edwin Ashworth's good answer, consider also unrestrained

Not restrained or restricted: a display of unrestrained delight

It can be used positively, negatively or neutrally.

Another term that can have a range of inflections is wild. While it often has a pejorative tone, it can be used in an admiring way

unconventional; fantastic; crazy: wild friends

It also can be used neutrally

Lacking regular order or arrangement; disarranged: wild locks of long hair.

A person who doesn't behave in a ritual manner or doesn't follow a fixed routine could be described as being easygoing. It usually carries very positive connotations, and implies the person is patient and tolerant, whereas disciplined can suggest intolerance and inflexibility.

Easygoing: Not rigorous, demanding, or stressful: "an easygoing life as a part-time consultant."