Go to the beach , Chanthaburi is not very far from the sea, there are several beaches within 20-30 kilometers. The best known is probably Chao Lao Beach. There are hotels and guesthouses that can accommodate you there.
Going to Chanthaburi Night Market held on weekends from 5 pm on Tha Chalaep Road on the banks of Somdej Phra Chao Tak Sin Maharat Public Park (GPS: 12.60314472,102.10688639). You can eat for cheap there.
Going to Gem Market , this is the great business attraction of Chanthaburi in Trok Kachang street. Although as tourists and non-professional, the gem market is not really the place to shop gemstones, it's fun to see how it works. And it is not far from the cathedral.
Boat tours on the river There are two companies offering boat tours on Chanthaburi's River. The rate is the same for both (600 THB recorded in March 2018). To book and have more information, you can call them at 093-3753992 or 061-5955866. Better to call them at least the day before.
Surfing on Chao Lao beach (only possible during the rainy season from May to October because otherwise the sea is too calm)
Making jewelry at Mejai Jewelry Studio on Sukhaphiban Road
Events and festivals in Chanthaburi
What better way to experience Thai cuisine and forge links with local than eating with a thai family in Thailand ?