"We will go on a date tomorrow."
Wouldn't it also be possible for just 私 to be implied? "We will go on a date" and "I will go on a date" seem to have some differences. "We" would imply you and a previously mentioned other person are going on a date, while "I" would leave the other person unknown, the focus being more on just you going on a date in general. For example, if someone asks what are you doing tomorrow, saying "we" might sound off because who's this undisclosed "we" that wasn't mentioned before.
Maybe I'm missing something in either or both languages (neither of which am I a native speaker of) but, though a minor difference, it could be worth distinguishing between singular and plural.
but there was another question where they wanted デートします for the same english phrase "go on a date"
I personally have found that when I stopped using the word bank and started typing Japanese on my keyboard, it really helped my retention and understanding. I have to think harder avour my sentences.
But it has been very frustrating, because Duo wants me to use a suru verb rather than the polite form in some questions, I just get it wrong! There is no prompt, except to look at the word bank.
If they want us to practice a certain type of grammar they should say so. If they want us to be free to type the answer, both polite and informal forms need to be accepted.
In this case, ni indicates the purpose. http://www.punipunijapan.com/japanese-particle-ni-purpose/
I got this question the form of 'Mark the correct meaning', but there was no right answer. The closest, which is marked correct, is 'あした、わたしたちはデートに行く。' which doesn't make sense. I can't report it either because the only option is 'The English sentence is unnatural or has an error.'