We are using Building Foundations of Scientific Understanding by Bernard Nebel.
This book is very thorough but it requires a lot of preparation yourself so the lessons are very much what you make them. It's divided up into four threads which are loosely physics, chemistry, biology and physical geography, although there's some overlapping. They are sequential so you have to start at the beginning of each thread, but you are supposed to teach all the threads concurrently. This works well as you don't get stuck for weeks doing the same thing. There's a very active forum run by the author where you can ask questions about the curriculum and specific lessons.
Each lesson is a description of what to teach, the key words to understand (very nice and clear, there are usually only four or five per lesson, if that) and practical work to do. Beyond that it's up to you how long you spend on it and what format the lessons take. I've found the practical work is essential because just sitting down with textbooks is so clinical. So we usually start with something practical, do a lot of discussion where I try to use all the key words for that lesson and finish with a written narration summarising the lessons in a few short sentences, very much like dw's history narrations. It's quite daunting at first with the thought required to plan the lessons, but once you've read his description carefully it's obvious what to do. The boys usually enjoy these lessons (apparently I'm the "best science teacher in the world...").