Speak to the Doctor Chakwas in the Hospital's reception area and persuade her to rejoin your crew again.
When given the option of where to be let off, ask for the hospital. From here, head into the main reception area and look to your left for Dr. Ravin standing over a hospital bed. If you collected the Failed Experimental, you'll offer this to her and in return you'll be rewarded with 30 XP, 1,000 Credits and a new War Asset - Hanar and Drell Forces (not bad for giving someone a single gift).
Head to the hospital reception area and you'll overhear the receptionist and a patient arguing,; if you speak to them you'll garner an additional +2 Paragon/Renegade points. Now turn around - at the shop terminal you can buy an additional Medi-Gel capacity for 6,000 Credits (bringing you up to five if you bought the upgrade on your previous Citadel visit). If you're injured colleague is Kaidan then buy the Peruvian Whiskey (it's the 'correct' gift to buy him), if Ashley is your squad-member then buy her the Collected Alfred Tennyson poetry collection for additional Paragon points.
Go through to the next area and you'll start to overhear conversations; if you walk around the various lab rooms, each conversation is the opening to a new side-mission that'll reward you with credits, war assets and more.
The Lead Scientist in the side-room opens up the 'Ismar Frontier: Prototype Components' mission, where as the Asari Scientist opens up the side-mission 'Citadel: Biotic Amp Interfaces'.
Citadel: Purgatory Bar
Look for a woman called Alison standing near the VIP bar with a datapad in hand; if you listen in on her conversation you'll open up a new side-mission 'Citadel: Improved Power Grid'. Inside, you can run up the stairs and on your right you'll find Aria T'Loak who will ask you to unite three Mercenary groups together to aid in your war efforts against the Reapers. This conversation unlocks the three new missions: Aria: Blood Pack, Aria: Blue Suns and Aria: Eclipse.
At the bar itself you'll find your colleague James propping it up; if you initiate a conversation with him you can offer to buy the other soldiers a drink to prove you're just like one of them. A Renegade prompt will appear that you can hit, offering you +2 on your reputation. You can even take a spin on the dance floor if you approach the top right-hand corner.
Citadel: Presidium Commons
Hang a left, run up the stairs and look for the two civilians sitting on the sofa to your right. If you talk to them, you'll be rewarded with a new War Asset - Citadel Defence Force and +2 reputation points. Walk to the upper-left corner of this area and you'll overhear a conversation unlocking a new side-mission 'Irune: Book of Plenix'.
Run down into the corridor behind you and just past the opening on your left there's a Salarian Businessman who you can talk to and earn +2 Reputation points and a handy War Asset - Alliance Fifth Fleet. Nip outside and run down the stairs. On the right-hand side you'll hear a couple arguing; if you intervene you'll be rewarded with +2 Reputation points and an improvement to your War Asset - Citadel Defence Force.
Sitting at a seat by Apollo's Bar you'll find Liara; chat to her for +2 Reputation and an invite to speak to Barla Von (who can be found standing behind a desk near where you spoke to the two civilians that were sitting down). Chat to him for a new side-mission, 'Citadel: Barla Von', where you need to evacuate some mercenaries to get them to join your cause. You'll also get +4 Paragon points into the bargain.
Back outside, go down the next flight of stairs to the 'Meridian Place' where you'll locate the newly mobile EDI standing outside. Chatting to her will reward you with +2 Reputation points after an... interesting conversation. The 'Nos Astra Sporting Goods' terminal allows you to buy some weapon upgrades (ignore) or the new Collectors Armour that provides +20% to Shield Regen Speed, Shields and Health for a cool 50,000 credits. Nip up to the shop (located up the stairs to your left) and speak to the clerk here to cut a refund argument short and get +2 Reputation points in return.
Aria: Blood Pack
Make your way upstairs to the C-Sec quarters where you can pick up a new side-mission, 'Citadel: Heating Unit Stabilisers' by listening in on the conversation occurring outside on the balcony. Run past him, up the stairs then look for the stairwells near the illuminated posters in the tunnel. Walk up to the left-hand one and head into Narl's office for an interesting conversation.
Agree to let Narl pretend you've been captured and you'll discover an interesting secret, along with be rewarded with 210 XP, +5 Reputation, +2 Paragon/Renegade points, 5,000 credits and a War Asset - Terminus Fleet. Nice haul.
Aria: Blue Suns
Have a natter with Darner Vosque (near bay E28), state you won't kill the general for +4 Reputation points and then make your way to the Presidium Commons. Run down the stairs near to the bar and you'll find the general sitting on a bench. Agree to help the general to acquire his arms via another method, then run up to the Aeghor Munitions shop nearby where the clerk Kannik will inform you that he's after an artefact if he's to offer his wares to the general.
Jump back onto the Normandy and in the Kite's Nest galaxy, go to the Vular system, scan the planet Vana and send down a probe which will reveal the artefacts you're after. Head back to the clerk and he'll agree to givie the general access to his best weapons, ending this part of Aria T'Loak's mission and rewarding you with 5,000 credits, + 5 Paragon/Renegade points and an improvement to the War Asset - Terminus Fleet.
Speak to Commander Bailey in his office and ask to see the prisoner in question. From here, make your way back to the C-Sec reception desk in the Presidium Commons. Chat with the prisoner and tell Bailey you'll find another way to solve the problem.
Make your way to the Docks: Holding area and near to where you chatted with Darner Vosque you'll meet up with Seyn. Persuade him to take control of Eclipse for himself and you'll have completed this part of the mission earning yourself 5,000 Credits and a boost to the War Asset - Terminus Fleet.
Citadel: Docks Holding Area
Upon entering, listen out for a conversation by the desk to your right; intervene and you'll get +2 Reputation points for your efforts. Before speaking to Darner Vosque, you'll find Garrus chatting away in the cul-de-sac just to the right. Have a chat with him for +2 Reputation points. A few bays up to the right there's a preacher - if you've collected the Pillar's of Strength Artifact from planet Khar'shan in the Harsa solar system you'll receive a sweet 15,000 credits, a War Asset - Batarian Fleet and +5 Reputation points.