There are lots of things to spend you money on, There are many tourist stores and mercados. Some things you might not think of are dentists and eye doctors . If you don't have insurance it is much much cheaper to get some good dentistry work done while you are here. Glasses and Contacts are also cheaper here. Another thing is getting custom clothing made while you are here.
Mexican tailors are very good and very cheap.
If you have a favorite shirt or pair of pants bring it with you and the tailor will copy it exactly. If you are staying with a Mexican family ask them for a referral, if not there are some good tailors just to the left of Cortez's palace, from the Zocolo walk down past Charlies bar, next street turn left. Check out the food page for wine suggestions. They also have a great tequila and wine selection. Golf Club Cuernavaca.