"今天晚上我有事" should be acceptable, too, right?
I think so too, I reported it just now.
EDIT: Still not accepted - reported it again. 2018.10.20
Still rejected - 2019.04.22
These latter exercises are really neglected. I guess you get what you pay for
Accepted now! 5.10.2020.
It can also be said "我今晚有事。"
So I guess 事 has the same meaning as 打算?
事 means "thing" or "affair". It's a really generic term. 打算 litterally means "to hit a calculation", and thus "to plan". They are completely different things: the former is a noun while the latter a verb.