Learn Romanian: Vizităm parcul acvatic astăzi. - We are visiting the aquatic park today.

Romanian doesn't have a separate present continuous tense (like "are visiting" or "am going"). The single present tense verb vizităm translates to both "we visit" and "we are visiting" depending on the context.

Parcul has the definite article meaning "the" attached to the end. How do we know to use -ul here?

Parc is a neuter noun (un parc, două parcuri). In Romanian, neuter nouns take the same definite article endings as masculine nouns in the singular, which is usually -ul for words ending in a consonant.

Could I put the adjective first and say acvaticul parc?

It would sound highly unnatural here. In Romanian, descriptive adjectives almost always come after the noun they describe. Putting the adjective before the noun is mostly reserved for poetic emphasis or a few specific adjectives like bun (good) or mare (big).

I often see azi for "today". Is there a difference between azi and astăzi?

They mean exactly the same thing and are used interchangeably. Astăzi is just a slightly longer, slightly more formal version of azi, but you will hear both used all the time in everyday spoken Romanian.