I was a night auditor (see a night in the life of a night auditor) at two different hotels on the Central Coast of California (specifically, San Luis Obispo and Pismo Beach) in the mid-1990s. While some things (mostly the technology) has changed, most things are about the same.
It would really help to know where you will be traveling, because California is such a diverse and huge place (larger than many states and even entire countries!).
Weeks 30-32 are July 22-August 11, which is the middle of summer. That's better for finding a room than than June. From late May through mid-June, you have a lot of formal dances and graduations. If those events hit around the same time in an area, you may find that all the rooms have sold out in the entire region. For example, the graduation weekend for Cal Poly SLO (a top university) will result in sold-out rooms from Paso Robles (nearly the northern border of SLO County) through Santa Barbara (middle of the next county south), which means you might have to drive well over an hour to find a hotel room if you did not plan ahead. I know this, because I had to literally call every hotel and motel I could think of to try to find a family a room that weekend. I finally did, but it was in Ventura (1.5 hours south).
Night auditors are great resources. Especially on weekends, we keep track of (or at least generally know) which hotels have vacancy. If you arrive late, ask the person at the desk for recommendations. You might pay more (usually the larger, more expensive hotels have vacancies when the smaller ones sell out). I also found that some B&Bs (bed and breakfast) had rooms left when the mainstream hotels were full. I sometimes had guests call me back to thank me for "upgrading" their vacation to a B&B (which they would not have thought of on their own).
Bear in mind that you will be in about the warmest (hottest) part of the year, especially if you are visiting Southern California or the Central Valley. Temperatures may reach 100F (30C) or higher (especially in early August). The deserts will climb even higher, but drop to chilly temperatures at night. However, along the coast you will likely experience cool, foggy days in the 60-68F range. In the Sierra Nevada mountains, days may be quite warm (80F), nights cold (40F), with sudden thunderstorms or even rain. That also is fire season, so you may find areas you want to visit closed or undesirable due fires or heavy smoke in the air. The countryside will probably be brown and dry looking--quite different from the lush green fields that you would see now (in mid-March).
Be sure to pack for a variety of weather conditions. Most hotels have laundry facilities on-site (coin operated washers and dryers mostly, with upscale establishments offering dry cleaning), so you can get by with fewer clothes, but more variety. Layer your clothes. Bring tee-shirts, khaki pants (like Dockers), a polo shirt or blouse, a nylon windbreaker, and, depending on where you travel, shorts, a long-sleeve dressier shirt, and maybe a medium-weight jacket. If you plan to visit high-end restaurants or night clubs, or if you will be visiting the San Francisco Bay Area, you might want to bring a complete set of dress clothing. Shoes can range from sandals/flip-flops, to sneakers or running shoes with good traction (for the beach or hiking), to casual dress (for evenings and dressier places).
In the middle of summer, you probably do not HAVE to make reservations except on the weekends or if you plan to stay near the ocean, or a major attraction (Disneyland, Napa, San Francisco, Sea World, etc.), or an airport. If you are visiting a small town, and there are limited hotels, I'd also recommend a reservation.
Weekdays (Sunday night through Thursday night) usually have lower occupancy rates, but, again, local events or conditions could quickly sell out all the available rooms.
Most hotels will provide you (if you ask) a complimentary 6pm (or 4pm if they are stingy) hold for no charge, no credit card required. They know that they can almost certainly let a room between 6 and midnight, even if you do not show up. Be careful about giving out your credit card, because they WILL charge you for one night if you don't show and don't cancel by the required time.
Note that a reservation guaranteed with a credit card is not a 100% guarantee that you will have a room waiting for you. Many hotels offer their front desk staff incentives to fully occupy the hotel on weekends. That means a person staying in every room, even if there are no-shows! It comes down to the front desk staff making guesses on how likely someone is going to show up or not. If you are going to be late, let the hotel know! If it is after 8 PM, call the hotel to let them know (or remind them) that you are going to be late. If it is after 10 PM, call them again. If it's after midnight and a weekend, call again. Seriously. You will be charged, of course, but at least you will have a room whenever you finally drag in.
Finally, when you are looking for a room, search the surrounding areas. Some nice properties are in slightly out of the way locations. Some that are in the unincorporated parts of the county, but very near a city, have an address listed in another city that may not sound nearby. Again, ask people at the hotel (night auditors, concierges, front desk staff, on-site property managers/owners, and even other guests) are great resources. I learned many interesting things from chatting with guests. I quickly learned from them which restaurants were good (and not so), and I passed that back along to my guests.