Cullen Bay has a range of restaurants which are open for evening dining.  Several offer views across the marina to the boats and upmarket bayside houses on the Bay.  Some restaurants are reasonably priced but offer good local seafood, e.g. you can get  the ubiquitous "battered barra" (barramundi) for around $20.  The bay can be reached by public bus, the reasonably recent number 14 goes from the bus station in town to the Bay in about 10 minutes.  The disadvantage of the bus is that the last one back to town was at 8.40 pm; but if you want to eat early and don't have your own transport, it is an option.

Cullen Bay is also where you can find various harbour tours, again at reasonable prices, and it would be a pity not to take one of them to see Darwin from its large harbour, "Bigger than Sydney Harbour" is constantly spruiked.  So size still matters in Darwin.