I do pikes on a swiss ball, sideways planks on a swiss ball, back bridges on a swiss ball, "upside down" pikes while laying on my back, also do one where I lay on my back and hold somebody's ankles, swing legs up to them and have them push them back down. I also "walk" on my hands in the plank position with my feet on the swiss ball. I do this forwards, sideways, backwards, etc while trying to keep the core perfectly straight.
I try to avoid situps, crunches, they are a waste of time I think. I also avoid planking on the ground. Getting your feet up on a ball is way more engaging.
Another one I do is put my feet onto a rowing machine's seat while I am in the plank position (hands on the ground, torso 90 degrees to the machine), then work my legs side to side, really engages the whole core.
If your goal is to improve form for shorter distances (800/1500), consider form drills 2x a week rather than a ton of core work. Maybe 1x per week (20 minutes) of core work and 2x per week (20 minutes each time) of form drills. Also really pay attention to form during all your speed work and have somebody giving you feedback on form during your workouts.