To get easy caps, go to the Strictly Business quest and capture Susan Lancaster to collect the reward. Then speak with her in the slave quarters part of Paradise Falls. You will need to pickpocket a key from one of the slavers to enter slave quarters. When she asks for help, choose the option "You're a slave. Deal with it.". When you return to grouse he will think you captured her again and reward you. Repeat as desired.
Note: Try this only if you want to take the "evil" path. After agreeing to help Mr. Burke nuke Megaton, go into every single house and steal everything (any little piece of junk you can carry). Then sell it all to the local shops (Craterside Supplies, The Restaurant, Moriarty's Saloon, etc.). If a particular house is locked or you need a key to get into a safe or something of that nature, just kill the owner of the property and get the key from their body. Kill everyone in sight, especially the shop owners because you can get access to their entire inventory, plus the caps that you have already spent on them. Kill every one and collect everything to leave Megaton empty. You should have at least 600 caps. Remember to sell some of the things you gather to the shop owners because you cannot fast travel when over encumbered. This can be done in pretty much any town you visit. To avoid getting over encumbered, get all the supplies that you can possibly carry, then drop the rest just outside of Megaton. Then fast travel to Rivet City or some other marketplace, swhat you have, and return to Megaton to repeat the process.
Save all pre-war books you find (not scorched or ruined) and go to the Arlington library. At the southeastern corner is an NPC that will buy those books for 100 caps each. You will get 10 experience points for each book you sold.
Do the "Riley's Rangers" side quest. Once you rescue Riley and escort her back to her base, she will ask if you want to help her out by mapping the area. Do not tell her that you will help her just yet. Instead, first reach level 20 and use the "Explorer" Perk. Once you have revealed every location on the map, go back to her and tell her that you will help her out, then tell her you have got some new locations for her. Depending on whether or not you have already done it, you can make upwards of 2,200 caps.
This process is very short. Set the difficulty to "Very Hard", then get a very strong follower, such as Fawks or Charon, and travel to the Citadel. Travel the exact path as you did when you did the "Taking It Back" quest . There will be a fair amount of Enclave personnel; let your followers take out the trash opponents. All you have to do is collect the armor and weaponry, repair it, then sell it. The giant robot will be at the end of the road, assisting you for the kills. Note: This will only work if you have activated the quest "Taking It Back".
This trick requires a good amount of patience, but it works on patched versions of the game and will not crash it. First, own a house in Megaton or a Tenpenny Suite. Talk to the butler robot and repeatedly tell him you are thirsty to receive purified water. After about seven times, he will say that his condensation condensers need to recharge. Sell the water for caps or give it to a beggar for good karma. Wait one week and ask for water again. He will give you seven more. This can be done indefinitely. Since you have to wait a week, all the shopkeepers will have enough caps to buy the water.