Prior to working at Disney, I've only been to Disneyland twice and apparently to DisneyWorld once (I went as an infant with my parents SO IT DOESN'T COUNT). After becoming an employee at Disney Animation, I don't really know how often I've gone to Disneyland but I literally basically went every other week. Most of the times just by myself because it's fun (no waiting with single rider lines and I get to go on Grizzly River Run as much as I want!) and to collect pins with my oh so delicious discount.
Here are some basics that I tend to keep to when I take people with me. Also this is for both Disneyland and California Adventure and attempting to do both parks in one day.
I'd recommend to try arriving at 9am or by 10am.
First thing that you want to do is head on over to Tomorrowland and grab a fastpass for Hyperspace Mountain. It's definitely one of the most popular rides and can have lines up to 2 hours or more long. You can either walk there or I recommend taking the monorail. I like taking the monorail as there isn't really a line at the ticket entrance unlike the main Disneyland entrance and you get a nice scenic view as the monorail drops you off right in Tomorrowland.
After this initial rush to Tomorrowland for Hyperspace Mountain it's a bit more relaxed. You can do some rides in Disneyland or maybe hop on over to California Adventure.
Fastpasses I would recommend grabbing are Hyperspace Mountain, Splash Mountain, Grizzly River Run, and Radiator Springs Racers, Star Tours, Indiana Jones Adventure.
Maybe Haunted Mansion. Usually it's not too bad, particularly since the ride is on a continuous belt. However, if you're going during the holidays and they have Haunted Mansion all redecorated with Nightmare Before Christmas, you will want a fastpass and it is a ride I definitely recommend.
There are a few attractions that have disconnected passes so you can grab it whenever and still get something else. World of Color, Buzz Lightyear Astro Blasters, and the Hyperior Theater (currently performing Frozen).
Important notes! Disneyland and California Adventure fastpasses are disconnected so you can get another fastpass at the other park while you wait for the cooldown of your current one.
Additionally, if someone is unfortunate enough to lose his or her fastpass, do not despair. Simply go to the cast member and explain the situation and politely ask if you may have a +1 and usually they'll accommodate you.
Teacups! They're a Disneyland classic. The line is quite short and goes quickly. Some sources say different things about which cup spins the fastest but I try to go for the plain purple cup.
Jungle Cruise is a must. I love their puns. I quite like going on this ride after dark.
Big Thunder Mountain, Pirates of the Caribbean, Haunted Mansion, and Indiana Jones are all great rides and things you should try to fit in your day.
My favorite are the water rides so Splash Mountain and Grizzly River Run. Those do tend to get pretty crowded midday when the temperature rises so try to avoid that if you don't have a fast pass. Even if it's a bit later in the afternoon and it's not as hot out would be a great time to go. I would recommend going before dusk and the temperature completely drops though as with the sun out it only takes a few minutes to dry off.
Radiator Springs Racers is a really popular ride and the fastpasses usually go out really fast, even more so than Hyperspace Mountain. I probably wouldn't skip Hyperspace to go for Radiator Springs though. The main attraction to the ride is the race between two cars at the end. If you didn't get a fastpass, skip on over to the single rider line and you might be able to race your friends at the end!
The other worthwhile ride is Cars land is Mator's Junkyard Jamboree. It goes around in circles and you sit in the back getting swung around!
Tower of Terror isn't really my favorite ride but something I'll do with people if they want. Particularly is that they will be closing it down for the Guardians of the Galaxy ride so now is the few last chances you'll get to do it. The line is usually none existent (unless going to late night check-out) and a fast pass really is only used to skip the first part to get to the elevator but once up everyone is in the same lines so not even useful.
Toy Story Midway Mania is really good and I think a lot better than Buzz Lightyear Astro Blaster. Unfortunately there's no fast pass and the line is always kind of long so do this if you're just waiting for a fastpass ride or waiting for cooldowns.
California Screamin' is a pretty great roller coaster. The line goes pretty fast so no need for a fastpass, too!
Soarin' Over the World is kind of just okay. It's just a giant projection show and the line can take forever as you have to wait for the previous show batch to finish. I strongly do not recommend doing single rider for this. If you must, just go in the normal line as there aren't really seats to fill so you end up standing in line for longer.
Rides I don't mention are ones that I feel have lines too long, not worth it, or both.
For actual meals I prefer going over to California Adventure. At the pier you have Boudin's and you can get a delicious soup or salad in a bread bowl and I find it very filling. The ribs over in Flo's V8 Cafe in Cars Land is also really good and something that I highly recommend. You can also get beers, margaritas, or vodka with lemonade/limeade in California Adventure!
Snacks-wise, other than the cheese cones at the Cozy Cones Motel in Cars Land, I would hop back over to Disneyland. There's the famous hand dipped corndog car. Just get the corndog without the chips or apples and they knock $2 off. My favorite place is the Mint Julep Bar over in New Orleans Square. The mint juleps are amazing and so refreshing. Get some Mickey Mouse beignets while you're at it! Don't forget a Dole whip! There's usually a long line outside that goes around the corner but there's a second line inside past the gate, too! It's a bit shorter and you get to see the animatronics while you wait.
One of my traditions is at the end of the day, before leaving the park, I like to stop by Jolly Holiday Bakery for something sweet, I usually get the special season cupcake (which are always amazing), and an abuelita (hot chocolate with cinnamon and spices).
Some people love the shows and some people see it as an opportunity to go on rides while there are less people in line. Up to you.
Mickey Mouse and the Magical Map is currently being performed at the Fantasyland Theater. Quite fun with lots of singing and dancing. Stitch actually got the loudest cheers when he came out on stage, over any of the other princesses.
There weren't great review about the Frozen live show at the Hyperion. Some saying the the singing was off. When I went, it was quite a bit later than when it opened, and I thought it was pretty fun and great. It is a giant crowd though so you will want a fastpass or to be there waiting an hour or two in advance.
I loved Paint the Night Parade. Apparently it's back! Go to this if you can and if you want.
Fireworks in Disneyland and World of Color in California Adventure every night was apparently only a 60th Diamond celebration thing. They still do this on select nights so it's worth taking a break and seeing.