I just really feel like beating someone up without getting a bounty.

Talk to people until you find someone that boasts about how strong they are, then chose an appropriate response. As far as I know you can only brawl with certain people.

Also try not to hit the people watching, i've done that a few times.

what tisamon said.
sadly it is with only certain people.
i wish there was a random encounter where someone will run up to you and challenge you to a brawl

Luckily I haven't hit anyone watching. Someone said that once they were in a brawl and a guard came up and said in a joking voice, "Stop right there criminal scum!" Has this happened to you because it hasn't happened to me and I've fought people in front of guards?

Just be aware occasionally you'll get an npc that will actually FIGHT you, not just brawl with you. Happened to me in a small town inn with a random Redguard warrior in full dwarven armor.

I'm glad I taunted him though...it was quite the epic fight. Each of my hits barely hurt him because 1) His armor was upper tier 2) His Heavy armor skill was high.

Bastard didn't think I was as tough as him. He thought wrong.

I remember way back before the game was released it was said that you could have people come up and challenge you to a duel, but haven't heard of this happeneing yet though.

i heard some people doing magic duels but thats it.

Had this, but it wasn't said in a joking voice.

you could also try farkas.
he gives out the hired muscle missions.
wont work for me because he always wants to fight a dragon,but i dont.

I accidentally punched a guard in Markath, and he got stuck following me. The guy I was fighting was sent to beat me up because I robbed someone's stall, have no clue how they knew I did that though. No one saw.

When I say the guard got stuck following me, I mean that literally. He was following me like a companion... a pissed off, threatening companion, facing me as if he was about to attack me, but holding a torch instead. I accidentally punched him in the face when I was fighting the other guy (it was dark and a dude walked up with a torch in my face, wth).

I fast-travelled to Whiterun stables and he came with me, then he went out of combat mode, made some comment like "the roads aren't safe to travel," and then walked off. It was so weird I couldn't for the life of me figure out what happened. I assume if he had been holding a sword instead of a torch I would have had no choice but to kill him.