First, I checked out sticked list of recommended mods, but I can not see the thing what I am looking for.
I very like all first missions on sunny island, until first meeting with mutants.
All these stealthy actions with binoculars in jungle were really addicting.
I also very liked this sunny background.
Then game totally changed after first meeting with... mutants?!? (facepalm)
I played few more missions and stoped liking this game.
Stealthy rambo changed into typical run and shoot to scifi monsters.
So, are there some MODs that offer good missions similar to few first ones in original game?

Also later in the game, you will find missions without mutants:

Training: only mercs
Carrier: only mercs
Fort: only mercs
Pier: only mercs
Research: only mercs (except one mutant seend)
Treehouse: mercs and mutants in the first part, after crossing the bridge only mercs
Bunker: mercs and mutants
Steam: only mercs (or was there a small area where they fight mutants?)
Regulator: only mercs
Control: mercs and mutants
Rebellion: mercs and mutants, but mostly mutants
Archive: first mutants, later mercs
Cooler: mercs and mutants
Boat: only mercs
Catacombs: first only mercs, then mercs and mutants
River: only mercs
Swamp: only mercs
Factory: only mercs
Dam: first part only mutants, then mutants and mercs, only mercs at the end
Volcano: mercs and mutants

Nice useful list.
Good to use with console to jump between maps.

They did the same with Crysis and those stupid Aliens :(

I actually like the trigens for the challenge they provide, and it's especially fun when you have the mercs fighting them as well. I've been able to get mercs to chase me somewhere and then run past a place where some trigens are, then watch the fun begin :) I agree that the sunny missions with mercs only are sometimes more fun, but like the first reply said, there are plenty of missions even after the trigens are introduced that have mercs only. Trigens usually scare the crap out of me when they jump out of nowhere and sprint straight towards my face, another reason they're fun to have around XD Tests your reflexes!