Learn Dutch: Tennis is een leuke sport.

In Dutch, adjectives usually get an -e ending when they come directly before a noun. Because sport is a de-word (de sport), the adjective keeps its -e ending even when preceded by een. If it were a het-word, the adjective would drop the -e after een (for example, een leuk spel).

Can I tell from this sentence whether sport is a de-word or a het-word?

Yes, you can! Because the phrase is een leuke sport, the -e on the adjective gives it away. If sport were a het-word, the adjective wouldn't have an -e after een.

Does leuke mean "nice" or "fun" here?

It translates well to both! Leuk is a very versatile and common Dutch word used to describe things that are enjoyable, pleasant, or fun. Calling tennis een leuke sport means it is an enjoyable and fun sport to play.