May to September is the rainy season and will ensure a wet visit. Thailand is a tropical country so bring clothes appropriate for the weather. Bring sunscreen and a hat to protect your skin. Shorts are appropriate but when visiting temples wear long pants.
Accommodations are not a problem with plenty of places available for all budgets. Chiang Mai is a relatively safe place although tuktuk drivers are notorious for scamming their passengers by offering overpriced “sightseeing tours”.