well, I know of a few, mostly older games were easyier, such as Zootycoon, orginal or complete collection, there is a tool out on the interent, or at least there used to be, called APE. animal project editor. it made modding the game so easy total noobs could do it.
then theres insaniqaurioum deluxe, I don't have it any more, never tried to mod it, but some people have, appearantly is wasn't so hard.
then GTA san andreas, but I don't know how mod that, its alot harder then these other games. but there is a tool that lets you change the pantjobs on certain cars, mostly the lowriders.
but games on steam, cannot be modded unless it has steam workshop. GTA SA on steam requieres you to download a new exe, if you want to mod it. that may or may nto be possible any more.
A good game to mod is Garry's Mod but thats pretty complex, so I never tried it.