Alternatives to situps?

So, after more than 3 decades of dutifully doing daily sit-ups and crunches (standards, bicycles, with and without weights, etc.)a recent trip to the physio, due to lower back pain, has left me feeling right mardy as I've been told to stop these immediately as they're likely to be the cause of the problem.

Sadly most of my old 1970s PE teachers are probably no longer around for me to litigate ( :D ) . Recommendations for the best (none apparatus requiring),alternative exercises, that I could perform on the kitchen floor, etc. would be much appreciated.

Planks probably the best known and one of the most effective.

My personal favourite is this...

https://m.youtube.com/watch?v=2owf3NTjGzk

Can be performed alone by grabbing something behind you for stability.

An ab wheel (granted the wheel is apparatus but mine only cost 3 quid) the results I've had from mine are far greater than anything else I've ever tried, not sure how it'll sit with your back issue but I've had sciatica in the past and find that this doesn't bother it like sit ups do.

Lie on your back on a mat, thighs vertical, knees bent at 90 degrees so shins horizontal, hands under your pelvis to cushion it - bring your knees and pelvis up until your shins are vertical and your knees come close to your face, having kept your knees bent at 90 degrees. This puts much less stress on the back, but still focuses on the abdominal muscles. Variation: aim your knees to a point just to one side of your face, then the other. You may need to OK these with your physio before starting them. I do 2 x (10 normal, 10 right, 10 left, 10 normal) in a 2 hour session, but of course I built up to this.

That's really helpful. Thanks for all of the suggestions. Mountain climbers was also suggested to me which is a bit ironic given all the fell running :D