Depends on your definition of fun.
- Completely eradicationg the Legion and killing Ceasar in his own bedroom: Fun.
- Killing Yes Man over and over and over again cause he respawns every minute or so: Fun at first, gets old fast
- Raining explosive death on a large/medium sized group of ghouls/fiends/deathclaws/scorpions with a grenade machine gun: FUN! Never gets old.
- Knocking Raul out yourself, so you can listen to him ramble on about how Pinco Boy (or w/e it was) is there, or asking if he's dead, only to realize he's not cause you're there: FUn, also quite hilarious.
- Stealing Boone's Beret and Sunglasses, just to hear him ask for them back: Fun, and hilarious for a while.
- Returning to the strip to sell everything you pillage after killing every killable NPC in FNV, only to realize you have virtually nobody to sell to: Priceless.
For everything else theres Vault-Tec Card.