Kethane
It used to be that if you wanted to add some game to your space sandbox, Kethane was your main option. KSP's 0.22 update changes that slightly - thanks to a tech-tree focused career mode - but even now, Kethane's additions make for a much more detailed and involved campaign. It populates planets with the titular resource, and provides a series of new parts to help scan, mine and store it.
Kethane, while scarce, can be found buried beneath the surface of all planets and moons. Once scanned and successfully mined, it can be converted into fuel. That makes it a valuable resource for long-distance travel. With the correct setup, you're able to embark on missions to distant planets, gather up their kethane reserves, then convert it to fuel to power the next leg of your self-sustaining journey.
Install if: you want an intragalactic mining operation.
Kerbal Alarm Clock
Most people won't need Kerbal Alarm Clock until they've already spent several hours in the game. But as your plans to colonise the galaxy become more grand, the demands on your attention are greatly increased. Get ambitious enough, and you'll be managing a persistent fleet of satellites, ships and space stations, all in need of controlled burns, docking procedures, and orbital transfers.
Kerbal Alarm Clock is the most effective way to manage that chaos. You can create alarms based on maneuver nodes, apoapsis, periapsis, closest approach between two vessels, and a variety of other options, all of which are displayed in a small list of on-screen countdowns. Each alarm call can be assigned to its target ship - or a specific Kerbalnaut - meaning you always know what needs doing, and where. You can even set alarms based on Earth time, with messages like, "no seriously, go to bed now."
Install if: you enjoy spinning plates.
HyperEdit
If you still think MechJeb is cheating, take a look at HyperEdit. It is cheating. Install it, tap Alt+H, and you're given a menu full of options that let you tweak and edit the game. With a few clicks, you can teleport your craft to the orbit of any planet on the solar system, then use the landing options to gracefully touch down.
Alternatively, you can instantly replenish your fuel, obliterate a selected craft, or readjust Kerbin's gravity to make escaping its atmosphere unnaturally difficult. HyperEdit is a flexible toolbox that, when used without restriction, completely destroys the difficulty. With a little imagination, though, you can use it to create your own custom scenarios. It's as simple as popping an abandoned craft on a distant planet, and suddenly you've got the basis for a tricky retrieval mission.
Install if: you don't like trying.
Universe Replacer
Like Chatterer, Universe Replacer doesn't add new parts or features. Instead, it expands KSP's potential for atmospheric exploration by letting you change the game's textures to customise its look. By placing png files in its 'Textures' folder, you can create new surfaces for planets and moons, give ship parts a new paintjob, or swap out the skins of your Kerbals.
If, like me, your artistic skills don't extend much further than crudely writing on planets, the community have already done the hard work. A number of retextured files have been shared online, many of them catalogued by Reddit's r/KSPTexturePacks. If you're looking for some recommendations, Celestial Bodies Revamped provides a nice planetary upgrade, and these custom Kerbals have a lovely '50s feel.
Install if: you want prettier planets.
