I replayed Crack in Time within the last couple of years, and its still one of my favourite games of all time, I just love so many things about it, I really loved moon challenges. Was going to do ToD but I played the PS4 Ratchet instead, definitely down to play that again.
I think those games are old enough that you could literally just whack them up to 4K and 60 (originals are 30fps I believe) and it would be a massive upgrade with no redone assets (Maybe skyboxes and 2D elements could be touched up if they look particularly crusty at 4K), remake in new engine would be amazing obviously but thats probably too tall an order.
Doing that challenge in CiT where you can't be hit at all while you fight all the mini-bosses + other enemies together was such a fun challenge for me, you could even cheese it by using really upgraded guns but it was still tricky in parts even with that.
Finding the parts for the Ryno is always fun too, great reward for the end of the game where you can just smash everything in sight. The Ryno IV in ToD was really freaky with its red "death beams" and I loved how it was REALLY OP even for a Ryno, while the Ryno V in CiT was great with the classical piece that played when you fired it.