(1)This sounds interesting, but I have too
much work to do at the moment.
(2)Unfortunately, I've had a few things come up.
(3)I'd love to do this, but I can't.
(4)I can't at the moment, but I'm happy to
help you with something else later.
(5)That sounds great, but I just can't put one
more thing on my calendar for the next few weeks.
Let me call you …
(6)I'm sorry, but I have an emergency to attend to.