Suggestion, opinion
It seems to me that ...
I don´t really think that ...
I think it´ s true to say that ...
I mean, don´t you agree that ...
I don´t really believe that ...
I think that ...
The point is that ...
Are you doing anything on...?
Would you like to...?
Do you fancy (sth/doing sth)?
Why don´t we (do sth)?
Let´s ...
How/What about (doing sth)?
I was wondering if you´d like to (do sth).
It would be great if we (did sth).
Sure, why not?
Sounds good to me.
Great!
Yes, that would be (great/good/useful/excellent).
Yes, (I think) that´s a good idea.
That´s a great idea.
That´s a good / an excellent suggestion!
Yes, that´s probably the best option.
That would / could be fun / interesting.
Sorry, but I can´t.
Sorry, but I´m busy on...
Sorry, but I´ve got to (do sth).
I´m afraid I can´t.
That´s a good idea, but...
I´d love to, but...
I don´t think so, (because)...
Well, I´d rather (do sth).
Yes, but don´t you think it would be better to (do sth)?
I think (that)...
In my view / opinion...
I´m convinced that...
To my mind, ...
Personally, I think...
As I see it, ...
From my point of view, ...
As far as I´m concerned, ...
I don´t think ...
I don´t really know (if)...
I´m not sure (about/if)...
I don´t have a strong opinion about that.
Reasons
I feel this way because ...
I believe so because ...
The reason why I think so is because ...