Arrive at your chosen hostel (Sukhumvit/Khao San area) and grab the included breakfast or a nearby veg-friendly cafe; hostels often let you store luggage if room not ready.
If you want lively nightlife, stroll Khao San for cheap drinks, street food (many veg options) and backpacker atmosphere; otherwise rest at the hostel.
Full-day boat trip visiting Phi Phi, snorkeling spots and scenic bays; tours typically run 7:30am–5:00pm and include lunch (ask for veg meal when booking).