Ab / Core Challenge! Resurecting a zombie?

I've been letting the training slide for too long so I'm trying to climb back into the saddle here. I was thinking back to some sets of core exercises that I did as part of an "ab challenge" when first getting started with triathlon. Digging waayy back in my logs, over 10 years, I found some of them. I couldn't find the challenge thread but as far as I can tell it went something like:
Your choice of any core exercises
Week #1 3x 10 min., Week #2 3x 15 min., Week #3 3x 18 min.,
Week #4 4x 15 min., Week #5 4x 18 min., Week #6 4x 20 min.

I gave it a trial run last night. This was the first time I've done these in a long time so I started with a short set and went slow and easy.
2x (10 push-ups, 20 sit-ups, 20 bridged marching, 40 bicycles)
- marching: from a bridge position
(on back, feet pulled up near butt, raise middle to straighten from shoulders to knees)
hold the bridge, alternately straighten one leg, return it, then the other for 1 count.
- bicycles: on back, knees bent, feet raised and kept off the floor, cycle one leg out straight and back bent, then the other leg for a count of 1.

There should probably be some planks in there too.
I liked these exercises because they don't require any equipment. Feel free to suggest other good core exercises, or to modify your own times if this seems too easy or too hard. As I recall, the original challenge thread had some good links for example exercises and links are welcome here too.

This is not a team or rivalry type challenge, simply a personal challenge that you may choose to take on. I'm going to try to do this again, starting next week, September 7. It's just a 6 week schedule for anyone who wants to start later too.