Can we apply for extra money for the same project if we have over-spent or have good ideas about extra activities?

Your project will be funded according to what you said you would do in your application form and you will have a funding agreement to that effect. You will be held to account if you overspend and this could jeopardise the outcome of your project as you will not receive more funding than you applied for. If you want more funding you will have to reapply for funding incorporating your ideas for extra activities.