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The Den is a fairly low level place where you'll come to pick up some easy xp, a few quests, the NPC Vic and importantly, your wastemobile.
Tubby on the west side and Flick south of Becky's casino are decent merchants with which to upgrade your supplies. Beware the kids near the doorways of the establishments on the west side, as they'll attempt to steal from you every time pass, so check your inventory every now and again, and then steal back from the kids if they took anything.
You can be healed for cheap from Mom's diner on the east side of town by buying the rat dinners.
Metzger the slaver and his guards can be killed for xp and a subsequent reward from Becky when you feel up to it, however, finish Lara's quests first. I'd advise you to come back later and complete the assassination of Metzger due to your probable weakness at this point, then talk with the slavers to free them and Becky for your reward.
Den Quests & Opportunities:
Here is a list of some of the quests and other opportunities in The Den, each with a brief synopsis containing at least one way to solve it. The remaining ways, and possibly other minor quests I will leave up to you.

1. Collect money from Fred:

First talk with Rebecca in the casino and ask her about work. She'll mention Fred's debt. South of Becky's somewhere you'll find a 'citizen' named Fred. Listen to his story about his debt to Rebecca and offer to pay it for him, while still letting him keep some of the money. If you let him keep something, you'll be able to come back much later (several months) and he'll give you $2k and a lot of nice ammo.

2. Get book from Derek:

Becky's second quest is to retrieve the book she lent to Derek. Derek is in Mom's Diner and the book is usually on the east side of town in an outhouse on the north end. Return it to Rebecca for your reward.

3. Lara wants to know what is being guarded in the church:

Talk with Lara on the west side of town, and she'll ask you to find out what the gang by the church is guarding. Go to the east side of town, and on the south end, talk with one of the guards. If your speech is up to it, you can talk your way in. Examine one of the crates and then return to Lara for your reward.

4. Get permission from Metzger for gang war:

Lara's next quest (after #3) is to begin a gang war, but you'll need permission from the slaver leader, Metzger on the east side. Ask him for permission and return to Lara for some xp and a bit of cash.

5. Find weakness in Tyler's gang guarding the church:

After quest #4, talk to Lara again to receive this mission. Talk to Tyler outside the church and he'll reveal a party that evening where they should be getting very drunk. Report back to Lara for xp.

6. Help Lara attack Tyler's gang:

After quest #5, agree to go with Lara on a gang raid at the church. This can be a moderate challenge at this point in the game, but well worth it for you to do, since you can loot all the gang member's bodies when it's over. For the most part, stay out of the way if you feel you're not up to it, and collect the rewards afterwards.

7. Deliver a meal to Smitty for Mom:

Speak with Mom inside her diner on the east side, and she'll ask you to take a meal to Smitty over on the west side. Do so and you'll get some xp, a stim and a free meal (which heals hp).

8. Free Vic from his debt by getting his radio from his house in Klamath and paying Metzger:

Vic has lousy aim for an NPC but can be usually of more value than not. He's being held in the slaver's compound on the east side. After you've found his radio in his house in Klamath, talk to Metzger to get permission to speak with Vic, then return the radio to the man and speak again with Metzger who will demand $1000 for his release. Seems like a lot of money but it's worth it at this point in the game, and you will be flooded with money in the not too distant future from a surplus of tradeable weapons.

9. Sabotage Becky's still:

You'll need a halfway decent lockpick skill to complete this quest. First ask Rebecca what her drinks cost, and then go over to the east side to see Frankie at The Hole. Ask him why his drinks cost even more, and he'll request that you find out why her drinks are so cheap. Go back to Becky's and wait for the guard behind Becky to saunter over to the roulette table. Now's your chance to pick the lock on the door, and head downstairs, discovering a still. Report your finding to Frankie for two optional solutions: destroy her still (at which point she will no longer give you the time of day) or tell Frankie to simply buy his inventory from her, for the most xp.

10. Get car part for Smitty:

On the west end of the west side of town, find Smitty the mechanic. You'll need to refer to quest #3 in Gecko to find the requested fuel cell controller before he can install it for $2000. You should already have the optional energy-saving fuel cell regulator from quest #4 in Klamath if you've been following the walkthrough, which he can install after the controller for $750. Congratulations on getting your own mobile inventory stash! --the trunk of the car of course. To refuel it at any time, use the backpack icon when you hold your cursor over the car, select either micro fusion cells or small energy cells from your inventory and that's it. To see how much fuel is remaining, examine the car with the binoculars after holding the cursor over it.

11. Return Anna's locket:

At night, speak with Anna the ghost on the southern end of the west side of town, and she'll tell you about her missing locket. Talk to Mom in her diner on the east side, who will refer you to Joey the thug. You'll find him on the west side exit grid to the east side. Convince him to give you her locket and return it to her, at which point she'll turn to dust. Pick up her remains, and a nearby shovel off the bookshelf, and head back to the east side cemetery. Equip the shovel and use it on Anna's grave in the bottom row, then toss her remains in the grave for some extra xp!