Formal:
* Espero que esteja se divertindo. (This is the most direct translation and works well in formal situations)
Informal:
* Espero que esteja se divertindo. (This is still correct and understandable, but slightly less formal)
* Espero que esteja curtindo. (This is more colloquial and common among friends)
* Espero que esteja tendo um bom tempo. (This is a longer, but still informal option)
In all cases, you can also replace "Espero" (I hope) with "Desejo" (I wish) for a slightly more emphatic expression.
Here are some examples:
* Espero que esteja se divertindo na festa! (I hope you're having fun at the party!)
* Desejo que esteja tendo um bom tempo em Portugal. (I wish you are having a good time in Portugal.)
* Espero que esteja curtindo as férias! (I hope you're enjoying your vacation!)
Remember to choose the expression that best fits your audience and situation.