Inviting & suggesting --
-- Do you want to [...]?
-- How would you like to [...]?
-- Why don't we [...]?
-- I'd (we'd) be delighted if you could [...].
-- Let's [...](, shall we?)
-- Shall we [...]?
-- What (how) about [...]?
Accepting --
-- I'd love (be delighted) to
-- Thanks! I'd like (love) to.
-- That sounds great!
Refusing --
-- I'm afraid I can't.
-- I'm (terribly) sorry, but I can't.
-- I'd (really) love to but I can't.