Banks change cash and (sometimes) travelers checks, but casas de cambio (currency exchange offices) are usually quicker and may offer better rates.
A 10% tip is expected in restaurants (often automatically added to your bill in tourist towns such as Antigua). In small comedores (basic, cheap eateries) tipping is optional, but follow local practice and leave some spare change.
Homestays Better to buy a gift than give cash
Hotels Q10 per bag
Restaurants 10% maximum (if not already included)
Taxis Not customary
Trekking & tour guides Q50 per person per day (extremely optional)