If in doubt, the number one rule is don't drink the water, and that includes ice. If you don't know for certain that the water is safe, always assume the worst. Reputable brands of bottled water or soft drinks are generally fine, although in some places bottles refilled with tap water are not unknown. Tea and coffee therefore are usually safe to drink too, as the water has to be boiled. Milk should be treated with suspicion, as it is often unpasteurised.
Boiled milk is fine if it is kept hygienically and yoghurt is always good.
As Quito and many other parts of Ecuador are located at a very high altitude, many tourists experience a minor form of altitude sickness when arriving. Most travellers experience no more than shortness of breath, more need of sleep or minor headaches. If, however, one travels to very high altitudes, one might experience more severe symptoms, such as vomiting, fatigue, insomnia, loss of apetite, a rapid pulse or irregular breathing when sleeping. The best thing to do against altitude sickness, is to travel slowly and take it easy during the first few days to give your body chance to adapt.