How about another day？Maybe some other time.
“take a rain check”字面意思是“拿一张雨票”。短语起源来自美国的棒球文化，如果比赛时因恶劣天气而不得不终止，官方将发给观众交换券(check)，凭借此券可以观看下一次的比赛。
由此衍生出了 take a rain check，使用在将约定改期的情况下。
Thank you, but I have to take a rain check on your invitation. 谢谢你，但很抱歉我不能接受你的邀请，改天吧。
Would you like to have dinner with me tonight?你愿意和我共进晚餐吗？
I'll take a rain check! I've got stuff to do.改天吧，我有点事。
1、Thank you so much for the invitation, but I'm afraid I have a prior engagement.非常感谢您的邀请，但我已经有约了。
2、I'm very sorry, but I won't be able to make it. Another time would be great, though!很抱歉，我无法去。要是换个时间就太好了！
3、It's a lovely idea, but I won't be able to join you this time.好主意，但我这次无法参加了。
4.That's very kind of you, but …您太好了，不过...
5、Could I join you later? I just have to finish something.我可以稍后加入你们吗？我必须先完成一些事情。
6、Today's not so good - how about tomorrow?
1、I'd like to help you but I can't take on any new commitments at the moment, I'm afraid.2、Actually, I think this is something you could do really well yourself!3、Why not have a go yourself, and give me a shout if you need help?4、Sorry, but that's not really something I can do. Maybe Emma can help you.5、I'm terrible at that sort of thing! What about getting Tom to have a look at it?