After Enya is defeated, you'll stop at a tiny rest area on the way to Karachi with HP and SP fully restored. This rest stop has a vending machine and recovery items if you want to stock up before taking on Lovers, and the map has a random encounter with Assassin x3 if you wish to grind levels.
Leave the rest stop using the horse to continue the story. There are two ways to go about the Lovers fight:
Go with Jotaro. You won't be able to join the battle with Kakyoin and Polnareff and gain EXP, but you'll get bonus FP with Jotaro. In addition, if your protagonist is female, you'll get an extra scene after Jotaro is thrown out of the jewelry store, which itself will differ if Jotaro's FP is over 9.
Go with Kakyoin and Polnareff. As long as your Stand isn't Pixies, Napalm Death, Quicksilver, Caravan, or Mr. Big, you'll be able to enter the brain and join the fight against Lovers. If you fool around too long in this fight (like if you get whole-party berserked for a long time), it'll end automatically with no XP/GP gain after a short scene.
Afterwards, you can continue the story by entering the boat shop at the south of the map.
In the southwest corner of the map, a man is harassing a young lady. Talk to him, and Jotaro will punch him out. Afterwards, Jotaro will gain +1FP.
Souvenir From Karachi
Requirements: Kakyoin is in the party
If you talk to the shopkeep at the general store in the northeast corner of the map, he'll begin to wonder what kind of souvenir to bring to his family, but first, he decides to pick something out for you. He'll give you a Sparkling Mirror, an unlimited-use item that blinds enemies, and he'll gain +1FP.
Requirements: Joseph is in the party
Talk to the child blowing bubbles by the docks, and Joseph will begin to wax nostalgic about an old friend of his. After the event, Joseph's FP will go up by 1.
Requirements: Polnareff is in the party
Walk up to the door of the building directly to the right of the hotel, and Polnareff will start to complain about his stomach. You'll convince him to ask to use the toilet at a nearby house, and afterwards, you'll gain a whopping +3 FP with Polnareff.
At the reservoir, if your level's above 30, your SP is above 300, and your current day is under 30, you can help them find water to end the drought. After the event, you'll gain 1000EXP, +1FP with your party member, and -3 bad karma. In addition, if you triggered the event with Pixies, you'll also gain 10,000G.
There's a mansion to the northeast of town - enter, and as you progress, you'll be slashed for 20~30 damage by something invisible. These "danger spots" are set, and once you're slashed once, they'll remain gone until you leave that floor. Once you reach the 2nd floor, talk to the man in the bedroom to start a battle.
Note: Mansion entrance is just right of the Tummy Troubles building and just left of the carpet-selling Merchant
For maps highlighting all the danger spots in red, click the following links: 
In the bookstore in the southwest part of town, talk to the clerk, and he'll offer you a Video Game from Japan. If Kakyoin is in the party, he'll stop you from buying the knockoff, raising his FP by 1. If you go alone or with another party member, as long as Kakyoin hasn't already warned you, you can buy the game - talk to Kakyoin afterwards and you can offer to give it to him, or brag about it. If you give it to him, the game won't be removed from the inventory, and his FP will go up by 2. Brag, and his FP will go up by 1, as will your bad karma. You will need $5,900 or the clerk won't offer the game
If carried over to a New Game +, the Video Game unlocks the Dragon Question III minigame. See Japan for details.
Range: M. Appears if you enter Joseph's brain with Kakyoin and Polnareff. The drop is extremely rare - a 5% chance. Not strong, but it's best to avoid them due to the time limit.
3 at range M and 7 at range L - 10 in total. Defeating the fakes will cause them to regenerate. Despite their numbers, their power is high, so defense techniques will be necessary to weather the wave of attacks if your level is low. Weak to Fire and especially Light. The real Lovers is the one at the very top, normally second-to-last or last in the battle order (L distance).
BOSS: Lovers Clones
Range: M. Appears in a group with Burns x1, Bombs x2, and Spirits x2, with same stats as in Calcutta.
Murderdoll Lv. 4
Same as the ones in Varanasi.
Same as the ones in Varanasi. You probably get the idea by now.
Murderdoll Lv. 5
Same as the ones in Varanasi, but won't appear if you defeated Joey there.
Good ol' zombie, same as ever.
High HP, so bring someone with Ripple attacks if you're planning on running into these guys.
It's a zombie that's half-gone, not a half-zombie/half-human. That'd be pretty dumb.
OPTIONAL BOSS: Don Cherry
Uses guns and recovery techniques. Not by any means a pushover, but Dirty Work is the one to watch out for.
OPTIONAL BOSS: Dirty Work
Don Cherry's Stand. Generates Slash Energy which can inflict Bleeding status.
OPTIONAL BOSS: Slash Energy
Range: S. 3 appear at the start of the battle. Can still be attacked despite being invisible.