Sure, let's rephrase these sentences using "unless," "so," and "should."

1. If the sun shines, we will walk into town.
- Unless: Unless the sun shines, we will not walk into town.
- So: The sun needs to shine, so we will walk into town.
- Should: Should the sun shine, we will walk into town.

2. If he has a fever, he'll see the doctor.
- Unless: Unless he has a fever, he won't see the doctor.
- So: He has a fever, so he'll see the doctor.
- Should: Should he have a fever, he'll see the doctor.

3. If my friends come, I will be very happy.
- Unless: Unless my friends come, I will not be very happy.
- So: My friends need to come, so I will be very happy.
- Should: Should my friends come, I will be very happy.

4. She will fly to New York if she earns a lot of money.
- Unless: Unless she earns a lot of money, she will not fly to New York.
- So: She needs to earn a lot of money, so she will fly to New York.
- Should: Should she earn a lot of money, she will fly to New York.

5. We will visit the museums if we travel to London.
- Unless: Unless we travel to London, we will not visit the museums.
- So: We need to travel to London, so we will visit the museums.
- Should: Should we travel to London, we will visit the museums.