Can I just hand-wash my clothes in the sink?

You can, but in high-humidity climates, clothes often take 24-48 hours to dry indoors. Professional laundry is cheaper and faster.

Should I pack a rain jacket?

No. In the tropics, a rain jacket will make you sweat more than the rain will wet you. Carry a cheap, disposable poncho for downpours and let your skin breathe.

Packing for Tropical Humidity Without Smelling Like a Locker Room

Stick to lightweight, moisture-wicking synthetic blends or linen and limit yourself to a 40L backpack. Bring no more than 5 days of clothes and plan to do laundry at local shops every week.

Ditch the cotton. Cotton absorbs moisture and stays damp for days, which is how mold grows and chafing starts. Use polyester-blend 'tech' fabrics or 100% linen.
Implement the 'one in, one out' rule. Only pack one pair of bottoms and three shirts in your bag. Wear one outfit while you travel; keep the others packed in compression cubes.
Use packing cubes for ventilation. Use mesh-top packing cubes. Even if the clothes are dry, the mesh allows trapped air to circulate so your bag doesn't hold a 'stale' scent.
Plan for professional laundry. Don't bring a 2-week supply. Look for 'kilo laundry' signs in Southeast Asia. For $2-3 USD per kilo, your clothes will be washed, dried, and folded within 24 hours.

Can I just hand-wash my clothes in the sink?: You can, but in high-humidity climates, clothes often take 24-48 hours to dry indoors. Professional laundry is cheaper and faster.
Should I pack a rain jacket?: No. In the tropics, a rain jacket will make you sweat more than the rain will wet you. Carry a cheap, disposable poncho for downpours and let your skin breathe.