Should I book my whole trip in advance?

No. Travel in Southeast Asia is fluid. Book your first 2 nights in each new country and wing the rest so you can change plans if you meet friends or want to stay longer.

Is it safe to leave my passport in the locker?

Keep your passport and large cash in a hidden money belt on your body while in transit. When in the hostel, keep your valuables in a locker, but double-check that your padlock is high-quality (not a cheap plastic one).

Booking Hostels for Your Southeast Asia Backpacking Trip

Use Hostelworld for initial research and social atmosphere reviews, then check the hostel's direct website to compare prices. Always book your first night in a new city at least 48 hours in advance to avoid arriving without a bed during peak season.

Use aggregate sites for discovery. Search Hostelworld or Booking.com to filter by 'Top Rated', price, and location. Look specifically for 'Social' ratings if you are traveling solo and want to meet people.
Verify with direct bookings. Once you identify a hostel, go to their official website. Many Southeast Asian hostels offer a 'free breakfast' or a 10% discount for booking directly rather than through an OTA (Online Travel Agency).
Check the 'hidden' fees. Confirm if the price includes AC (air conditioning) versus just a fan. In Southeast Asia, you want AC. Ensure bed linen is included; some cheaper places charge an extra $2 USD per night for sheets.
Read the last 3 months of reviews. Ignore reviews older than six months. Look for mentions of 'bed bugs', 'loud construction', or 'unhelpful staff'. A high rating from 2022 doesn't matter if the management changed last month.

Should I book my whole trip in advance?: No. Travel in Southeast Asia is fluid. Book your first 2 nights in each new country and wing the rest so you can change plans if you meet friends or want to stay longer.
Is it safe to leave my passport in the locker?: Keep your passport and large cash in a hidden money belt on your body while in transit. When in the hostel, keep your valuables in a locker, but double-check that your padlock is high-quality (not a cheap plastic one).