Graphics mods!
I'm not a huge fan of the Firaxis-supplied terrain-art, especially the Jungle (which is sometimes difficult to distinguish from Forests), so I would recommend one of the following:
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@Sn00py's (greener) terrain (several different variants included with a Civ3Complete installation — or DL it from CivFanatics)
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@warpstorm's watercolour terrain is pretty stylish (again, included with Complete; I installed that one into my Sengoku Conquest folder)
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@Ares de Borg's more realistically textured terrain is also very pretty
I found the Firaxis Cultural borders difficult to make out on some terrains: I'm using
@Corn's borders instead (although I now can't find them in the downloads-database: the .pcx files are not associated with that username). The Firaxis rails are also quite ugly: I use
@Pounder's(?) thin rails (he also made some nicer Marsh terrain).
And for easier recognition of citizen moods in my cities (and on the F1 screen), I use
@Puppeteer's Popheads mod (just the basic one, without the nationality flags).
As for actual (epic-game) Mods and Scenarios, there are loads. Here are a few of the best that I've played:
@Yoda Power's
Giant Earth map (map only)
@Civinator's
CCM (and also RARR) epic-game mod
@AnthonyBoscia's
WorldWide epic-game mod
@player1 fanatic's
Patch Suggestions tweak-mod (.biq-only, rebalance of the basic Conquests game)
@Vuldacon's
Escape from Zombie Island scenarios
@Plotinus'
The Desert & The Mountain, and
The Rood + The Dragon historical scenarios (central Africa and Dark Age Britain, respectively)
@Meteor Man's
Star Wars The Mod Awakens
@WildWeazel's
Lord of the Mods (LotR mod)
@Nathiri's
Thelen Epres Mrel Nelthelrinae fantasy mod
@haluu's
Tides of Crimson fantasy mod
@KingArthur's
SuperCiv superhero mod
By clicking on a username above, you will be taken to the respective CFC-profile pages, where you can use the 'Downloads' tab to see their contributions to Civ(3)-modding.
Or you can have a look in the "Civ3 Creation & Customization" forum, "Completed Mods/ Scenarios" subfora, to see if anything catches your eye.