Hello
I currently have the fastest real time run of this game but it's easily improvable, especially in Sunday part 1. I started working on a TAS of this a few months ago but I kind of lost interest because I'm not that into it, I guess.
This lua script displays all of the hidden items in the game -
http://deanyderkheiser.net/marky/garfield.lua (change 0xB9E0 to 0xB8E0 in Saturday 2 & Sunday 2, thanks to Acmlm for making the script)
I didn't pick up all of them in Saturday and Sunday iirc but I'm fairly sure that I got all of em in Monday-Friday. I'll definitely get back to improving my run some time but not right now. I didn't put too much effort into optimising my run as you'll see by me picking up an excessive amount of health/ammo that I actually wasn't needed.
Edit: Here's my old
WIP of monday which has a few mistakes in it (like not grabbing pie and grabbing the second pair of speedboots from the front) but it showcases a few tricks;
Damageboost without taking damage: Jump into an enemy from below right after Garfield has done his low style judo kick and you should get push forward a tiny bit without taking damage (done at frame 390)
Health glitch: Pick up a rotten tuna when Garfield's HP is 1 and his HP will get to full which is 40. I'm not exactly sure if it's worth doing for a TAS but it might be worth it for later damageboosts.
Fast fall: Jump after walking off a ledge and you start to fall faster.
I think that you should try to crank up the entertainment value a bit more by killing more enemies if possible. It's also possible to glitch the game into displaying weird graphics and changing the music (demonstrated
here) but I'm not entirely sure how it works. That too could also make for some amusing stuff...