Nigerian passport holders can travel to several European countries for tourism or business purposes without a visa or with relatively straightforward visa procedures. However, visa policies can change, and it's essential to check with the respective country's embassy or consulate for the most up-to-date information. As of my last knowledge update in January 2022, here are 10 European countries Nigerians can often travel to more easily:

1. **Seychelles**: Nigerians can visit Seychelles visa-free for up to 30 days.

2. **Mauritius**: Nigerians can obtain a visa on arrival for a stay of up to 90 days.

3. **Georgia**: Nigerians can visit Georgia for up to one year visa-free.

4. **Serbia**: Nigerians can visit Serbia visa-free for up to 90 days within a 180-day period.

5. **Ukraine**: Nigerians can obtain an e-visa or visa on arrival for a stay of up to 90 days for tourism.

6. **Moldova**: Nigerians can visit Moldova visa-free for up to 90 days within a 180-day period.

7. **Albania**: Nigerians can visit Albania visa-free for up to one year.

8. **Kosovo**: Nigerians can visit Kosovo visa-free for up to 90 days within a 180-day period.

9. **North Macedonia**: Nigerians can visit North Macedonia visa-free for up to 90 days within a 180-day period.

10. **Bosnia and Herzegovina**: Nigerians can visit Bosnia and Herzegovina visa-free for up to 90 days within a 180-day period.

Please note that visa policies can change, and it's essential to verify the requirements and restrictions with the respective embassies or official government websites before making travel plans. Additionally, the purpose and duration of your visit may affect the specific visa or entry requirements, so be sure to plan accordingly.

The 20 countries with the coldest temperatures can vary depending on the time of year and specific locations within each country. However, generally, the following countries are known for experiencing some of the coldest temperatures:

1. Antarctica
2. Russia
3. Canada
4. United States (Alaska)
5. Greenland (autonomous territory of Denmark)
6. Mongolia
7. Kazakhstan
8. Iceland
9. Finland
10. Sweden
11. Norway
12. Estonia
13. Latvia
14. Lithuania
15. Belarus
16. Ukraine
17. China (northern regions)
18. Nepal (high mountain regions)
19. Bhutan (high mountain regions)
20. Kyrgyzstan

Keep in mind that the specific temperatures can vary greatly within these countries, and extreme cold conditions are often found in remote, northern, or mountainous regions.