10 other tasks were added to the project whilst adding content etc, new work... out of scope. This work should be either quoted for or set to an hourly rate, so you just charge for the extra time spent. What I would do is send an invoice for the full remaining amount. Then include 'addition fees' to the invoice as per extra work - out of scope time spent on the project.

Tread lightly here. Sure, they added tasks that were outside of the original project scope, but you should have, at the time of request, made it clear that these would incur additional fees. If you didn't state this or detail in your agreement, you might be out that amount. You may not have issues getting them to pay for these extra tasks, so I'd keep it outside of the invoices pertaining to the original agreement. Next time, DON'T do any work outside of scope without getting the okay from the client on the additional fees. And, have it in writing.

As derek lapp suggested, do not hand over any deliverables until payments for both invoices are paid.

And, regardless of delays on the clients' part, you cannot request payment until the work you agreed to do has been completed. Especially if it's merely for convenience on your part. Something you may try to do is to update the agreement with new timelines, clear wording on what's expected from the client (with dates), and the addition of the additional tasks performed along with fees. It's something I'd mention to them and then work on after they agree. This is something that will be beneficial to both of you.

Hope things work out and that you've learned something from this experience.