All day activites (if you have kids, or just an inner child):
Disneyland
Knott's Berry Farm

Mornings:
go kayaking around the Dunes in Newport
go for a hike in the Laguna area
get pancakes at The Greeters in Laguna beach (it is on the sand)
Stop by the local farmer's market in Corona del Mar (part of newport on the way to Laguna.) It's Saturday from 9am-12pm
Plum's Cafe on 17th Street (border of Newport and Costa Mesa.) A bit pricey, quite the locals hangout.
Then stop by Twice the style a few doors down for second hand designer clothing if you are into shopping.
Shirley's Bagels is also on 17th street. grab some delicious bagels on the drive to the beach.
Kean's Coffee is owned and run by the Diedrich family. They are there everyday. Most only Newport longtime locals know about it. Cool environment. Promelis is right next door and is great (but expensive) for picking up food for a picnic. It is a small gourmet market with an amazing deli.

Daytime:
If you feel like seeing the "stuffier" side of Newport, go to the Balboa Bay Club on PCH in Newport. (Think the TV show "the OC" type)
Go to Fashion Island and wander around. It's a shopping mall. Everything is a bit expensive though. Do some shopping grab some ice cream or pretzel and hang out and people watch by the one of the fountains.
Wander around Huntington Beach's Main Street. Parking gets really annoying, so get there early and bring tons of quarters. Watch the local surfers in Huntington by Main Street or go to 'the Wedge" in Newport.
Get sandwiches at C'est SI Bon (spelling?) in Newport off Riverside street and head up the hill to the park for a pretty view of the ocean.
Go to Catalina Fish Company on 17th street in Costa Mesa for fish tacos and beer
Go to Avila's El Ranchito off Placentia and Victoria in Costa Mesa for a Cadillac Margarita, guacamole, and tacos and enchiladas. (get the enchiladas ranchero style) Southern Cali is known for Mexican food and this place is as authentic as it gets.
Go to TK burger on the Newport peninsula.
Go to The Crab Cooker and get the scallops and shrimp. (make sure the potatoes are one of your sides.

In-N-Out Burgers when you are on the run and just want a good fresh burger
South Coast Plaza to gawk at all the designer stores
Balboa Fun Zone if you have kids. Take your rental car across the the ferry.
wander around Laguna on foot. (drive there of course)
SideStreet Cafe. Tiny little mom and pop joint off of Newport Blvd. It is already way too popular and crowded. Try the lemonade.


Night:
The Cannery is a very popular and nice restaurant in Newport
The Savannah Supper Club/House by South Coast Plaza.
Fusion Sushi on 17th street (on the border of newport and costa mesa) definitely a locals favorite
The bars on the Newport peninsula are quite a scene. Thurs.-Saturday. Popular bars are: Sharkeez, Newport Brewing Co., Malarky's, The District, The Beachball, Mutt Lynch's
to name a few. Think ages 21-35. Dress sexy but casual. No cocktail dresses or you will get funny lookcs
3 Thirty 3 is a better bar for the more upscale crowd. Ages here range from about 25-55. Gold diggers galore.
(Landmark in Laguna also)
Sutra is more of a night club. that is on Newport Blvd. A bit of a different crowd.

Careful where you book your hotel.  Try to stay in Newport Beach, Huntington Beach, or Laguna Beach. (careful about Huntington. It is not dangerous at all, but some areas aren't very nice, and you may be disappointed in "deal" you got on your hotel. Irvine is perfectly clean and acceptable. It's just that it's about 20 minutes to anything fun to do.
Avoid staying in Santa Ana and Tustin and areas of North Orange County unless you are staying somewhere you KNOW is nice, or right next to somewhere you are going to spend all day, like Disneyland. Not all of Orange County is nice. There areas up there that you need to watch where you park your car, how late you are out on the streets, ect. It is best to just not have to worry about it. Unless you are with someone from around here. They will most likely know what areas to avoid.
You have to drive to have any fun here. Everything is a "15 minute drive away, nothing seems to be a 15 min Walk away." plus people here are not very considerate of pedestrians.

With the exception of biking from the Newport Peninsula to Huntington Beach along PCH. That is fun.
If you have any choice. Stay where you are from 4:30pm-6:30/7pm Mon-Fri. If you try to drive anywhere, it will take you twice as long and give you quite the headache.
Rarely see anyone smoke around here unless it is at night around the bars. Careful. Places are very strict about that. Lots of dirty looks.