20C sounds about right as an average for Tunisia in January - I certainly wouldn't expect it to be much higher. The weather can be quite mixed at that time of year - expect to have some days when you can sit out in the sun but be prepared for the possibility of rain too. And it would be a good idea to pack a jumper or similar for the evenings as it could get cool of evening to be sitting out otherwise. Don't forget - Tunisia isn't actually that far from the 'toe' of Italy so don't expect the sort of heat you'd get in the Red Sea resorts at that time of year.
RE Cuba depending on when you went the humidity can be really high which speaking personally, I find always makes the temperatures seem much higher than they actually are. I've never found humidity especially a problem in Tunisia.
SM