We use Singapore's curriculum for science. Completely secular and heavy on critical thinking, but very light on "content" -- much more process oriented. That's exactly why I like it, but a lot of people see that and think "too easy".
It's not a hugely difficult curriculum either way -- workload is very manageable and it's easy to accelerate -- but it does require more independent thought than one would expect, and the math requirements track with the Singaporean math curriculum. So if you're using Singapore for math, I'd match the levels and just expect it to be a little on the content-light side. That does give us time for DS to do a science fair project, so it's not all bad.
