Pregame restaurant: "The Tin Fish." It's a mostly-outdoor, very informal patio-style restaurant with good beer and food. It's the closest I can find in downtown San Diego to a good pre-game atmosphere. Alternatively, you can stroll the Gaslamp District where there is an assortment of bars and restaurants, but the pregame atmosphere is almost always decidedly lacking. If you like a good dive bar, "Tivoli's" is also nearby and has good pregame feel and has a sorta Chicago feel to it. Petco itself: What makes the park unique is definitely the Western Metal building straddling the left field foul pole. Go up to the "Hall of Fame" grill which is kind of more like a sports bar. They have a patio on which you can sit and watch parts of the game if it is not rented out to some corporation. There's a lineup procedure to get out there for a few innings. Petco also has the "park in the park" with a kids-baseball field which is definitely worth strolling around. There's a HUGE screen on the back of the batter's eye so you don't miss much of the game if you decide to take an inning or two away from your seats. Petco has lots of standing room areas so it's also worth strolling around the concourse and checking out the various vantage points. ENJOY!

I've been to all six of those parks multiple times.  

First off, you don't want to do anything within walking distance of the Coliseum in Oakland.  If you feel the need to eat in the neighborhood around the stadium, I recommend the In-n-Out Burger on Hegenberger and not much else.  You're better off eating in Berkeley or San Francisco.

In San Francisco I think Sections 305 and 307 have the best view of the surrounding environs (plus you have a great angle on anything that ends up in McCovey Cove).  Check it out up there if you can.  The food at SBC tends to be very good.   There aren't too many places to eat right next to the park, but you can go a little farther afield (use MUNI if you have time) and find almost anything.  If you're sticking to the waterfront, there are a couple of good places in the Ferry Building; Taylor's Automatic Refresher is a good bet if you're looking for something relatively quick (think somewhat upscale drive-in food).  Fisherman's Wharf is one gigantic tourist trap.  If you find yourself in North Beach, Caffe Greco has fantastic dark chocolate gelato.  And if you're going to a night game, it will be cooler than you think.

Dodger Stadium:  as noted above, not much immediately surrounding the stadium.  The reserved level (blue seats) tends to have the roughest crowd.  I've seen fights in that level (including the largest stadium brawl I've ever witnessed, and I've seen quite a few) probably four out of the last five Dodger games I've been to.  If you're sitting there, be aware--particularly with the Mets in town.  (Mets fans get routinely pelted with garbage, etc. at Dodger Stadium.)   Also, everyone raves about the garlic fries in San Francisco--well, it's a lesser known fact that they serve the same ones at Dodger Stadium.  If you can find one of the stands that serves both the garlic fries and grilled (NOT steamed) Dodger Dogs, you've hit the jackpot.

Anaheim:  also not much within walking distance.  Lousy stadium food.  Much better baseball atmosphere in the lower level than in the upper level.

Petco Park:  in my opinion one of the most overrated parks in baseball.  It looks beautiful, but try to watch a game from the upper deck (broken up into three separate pieces) or the obstructed-view bleachers in center field and you'll see what I mean--it just doesn't work from a fan's standpoint.  (Can't see the field from the concourses, etc.)  Eat at least one fish taco, but the food there is nowhere near as good as it was at Jack Murphy.  (Which, I realize, does you no good.  Sorry!)

Chase Field/the Bob:  Pizzeria Bianco (within a few blocks of the stadium) is rumored to be one of the best gourmet NY-style pizzerias in the country and might be worth checking out.  I still haven't made it there, so I can't tell you firsthand.  Beware the back half of the upper deck, because you won't get any of the air conditioning.  If you're there early enough to watch them open the roof, it's kind of neat.  There is one concession stand by third base on the lower level where they sell the "big dog," which is simply the largest hot dog I've ever seen.  It's not always listed on the menu, so you may have to ask.

Let me know if you have any more specific questions, as I'm happy to share whatever knowledge I've accumulated!

Is located near Disneyland so there is so much to do there.  ESPN Zone in Anaheim was a great experience for me when I went to an Angels/Rangers game on Ring Night in 2004.