Dear Tom;
I am sorry to say I can't go to BeiJing tour with you next weekend.Because I have alot of homework this weekend.(随便写的突发事件）How about going there the week after next(让步说下下周去怎么样）If not. I wish you have a wonderful trip (如果你下周要去,我祝你有一次美好的旅行）
your friend
IlHua