1. Visit Ancient Khmer temples
The most are in the North-East of Thailand with the majors being "Prasat Phnom Rung" and "Prasat Phi Mai". My favorite is "Phnom Rung" nearby Buriram located on a mountain with stunning view over the Khorat plateau.
2. Visit National Parks
The biggest National Park is "Khao Yai" with wild life and beautiful waterfalls. But also in Loei province are 2 fantastic National Parks, "Phu Hin Rong Khla" and "Phu Luang".
3. Travel along The Mekong River
For almost 1.000 kilometer the mighty Mekong River is the border between Thailand and Laos with Indo-China markets in Nong Khai and Mukdahan, stunning views over the Mekong and of the beaten track places like Bung Khan and That Phanom.
Due to bordering to Laos and Cambodia it is a great gateway to Laos, Vietnam and Cambodia with international border crossings at Tha-Li, Nong Khai, Bung Kan, Nakhon Phanom, Mukdahan, Ubon Ratchathani and a few more.
5. Go to remote provinces
The area of Loei province is one of the less traveled areas of Thailand, while there are beautiful National Parks, waterfalls, century old temples and beautiful landscapes. Highly recommended!