To start off I'll talk about some grass related .ini edits you should tweak even if you're not using mods.
Skyrim.ini
[Grass]
iMinGrassSize=
Are the usual numbers you should use, lower means more grass and lower FPS
iMaxGrassTypesPerTexure=
This one vurts wants you to set to 7, it makes more grass appear. You can use a lower value if you think there is too many grass types rendering.
Skyrimprefs.ini
[Grass]
fGrassStartFadeDistance=
fGrassMaxStartFadeDistance=
These control how far the grass renders, I set mine both to 14000 and I think it caps at 20000.
Flora mesh replacers:
- Bigger Trees - Makes trees much bigger, I think it adds some too. This mod works with mesh replacing mods.
- Unique Grasses - Good mod that adds grass. Can be used with other grass mods.
- Skyrim Reforestation Project - Build your own forests and crash your own game! Likely great for save bloat and adding Immersive™ and Natural™ trees to your cities. Not quite a replacer but I thought I'd mention it since it's a very cool mod.
- Lush Trees & Grass - I highly recommend using the old mesh replacers as the new ones look retarded. I've re-uploaded two sets here and here. It's also worth noting that OpticShooter has actively removed the mods a couple of times and I have no idea why, maybe these old replacers are full of spiders or something. Even so they're the best mesh replacers in my opinion.
- Ultimate Lush Overhaul - I've always just used Lush Trees so my experience with this mod is limited. It's got an FPS hit though.
- Tamriel Reloaded - This is a texture overhaul that probably isn't to your tastes but what it does to grass is amazing.
Flora texture replacers & .esp's for grass:
LOD textures:
- AOF Tree LODs - I believe these are the definitive LOD tree textures.
Weather mods: