Most textbooks are arranged around some themes such as travelling, eating, sports etc. As Nicolas mentioned, when you define your goal, you can convert the activities into task-based ones. Showing them a model definitely helps and students get used to it over time.
In Belgium, most of the coursebooks are nowadays organized according to a task-based approach. The ones that don't still present chapters around one or multiple themes. If you take Meral's example: eating, then you should specify the specific lexical field of that theme like e.g. "at the restaurant". At the restaurant, one's non linguistic goal is to order what you want to eat, and this would be an example of a final task: "You're at the restaurant, here's the menu, order what you want to eat", the specific competence to carry out would be "being capable of ordering a complete meal at the restaurant". From that point it's the teachers role to define all that will precede the execution of the task: what the learner needs so that he would reach his goal (depending on your linguistic approach, be it focus on form: then specify grammar and lexis needed; usage-based: constructions that will help the learner: formulaic phrases, chunks ...)
There is almost no standard language lesson that cannot be translated into an interactive task based lesson with role playing. Language is task based, unless you are working on a very high level of theoretical study. Business English can be used for acting out business situations. Weather vocabulary can be used for acting out weather situations. Food vocabulary is the most fun of all. I once worked with a class to throw an end of the year party, beginning with creating committees for planning the event, doing the shopping (using newspaper food ads), the decorating, the cooking, the invitations, and finally a class sing along at the party that included Spanish songs and English songs that they presented to their familes. It was great fun and filled the students with lots of new vocabulary, pride, and satisfaction.
If you want to conduct close ended tasks, then following textbook is suffice, but if you wish to develop students' discursive skills through open ended tasks, engage the students with their self prepared materials.
I agree that textbook sequences based around the PPP paradigm can be adapted to fit a task-based approach. However, you need to look carefully at the activities and decide how interesting and motivating they are intrinsically and how they would work out once converted into a task-based approach, and what you can add to make them more interesting. I think merely changing the order so that the language focus comes at the end is not enough.
I also believe that you could devise your own tasks, perhaps trying out frameworks proposed by experts such as Jane Willis and Prabhu. Another question we need to consider is whether to use task-based learning to cover a predefined language syllabus or to deal reactively with the language that arises from tasks that are chosen or devised on the basis of their interest value and their potential for providing practice in the "four skills". A mixed approach, with some predefined language to ensure coverage of syllabus requirements and some work on the language that arises from the performance of the tasks can work quite well.