Restaurants
15% of the bill is standard. 

Spas
15-20% of the service is standard at most spas.

Taxis 
Taxi drivers do not expect to be tipped unless they perform some extra service such as carrying your bags or luggage to your hotel entrance, etc.  But almost everybody leaves them some. Usually the coins change. 

Ushers in movie theaters

One peso for showing you to your assigned seat. At he actual inflation rates, 50 cents is nothing compared to the ticket price.

Hotels

A minimum of one peso per baggage for taking luggage to your room. But with that minimum you are not gaining any further service. Better make the minimum a 2 pesos bill.