I was in Disneyland at the beginning of July and managed to be there during the hottest day Disneyland has ever had. The temperate hit a record 116 degrees that day and to say that it was hot is an understatement. 
That day was an unusually hot day but summer in Disneyland can easily be in the 90s and does get into the low 100s on some days so if you go between May and October you have to be prepared for Disneyland in the heat.  That day I was there aside as long as you are prepared Disneyland in the heat can be quite nice.
 The first thing you have to make sure you are doing when the weather is hot it drink plenty of water.  Disneyland has water fountains next to almost all of their bathrooms so I always make sure I take a few drinks every time I use the restroom.  If you bring your own bottle this is also a great time to fill your bottle or you can use the little filtered taps that some of the counter service places have like The Mint Julip Bar.  Another great way to fill your water bottle is to go into any counter service restaurant and ask for a cup of water. You can then dump the water into your bottle and continue with your day.  Sometimes they even have a tray of waters out for you to grab when the weather is especially hot.
The first thing you have to make sure you are doing when the weather is hot it drink plenty of water.  Disneyland has water fountains next to almost all of their bathrooms so I always make sure I take a few drinks every time I use the restroom.  If you bring your own bottle this is also a great time to fill your bottle or you can use the little filtered taps that some of the counter service places have like The Mint Julip Bar.  Another great way to fill your water bottle is to go into any counter service restaurant and ask for a cup of water. You can then dump the water into your bottle and continue with your day.  Sometimes they even have a tray of waters out for you to grab when the weather is especially hot.
In California Adventure one of the best places to enjoy the air conditioning is inside the Animation Academy.  You can meet Ana & Elsa if you are in the mood for characters, see Turtle Talk with Crush, visit Beast's Library or draw a character.  If you do all 4 things it will easily give you an hour in the air conditioning.  Another place to enjoy the air conditioning is in the Frozen show.  The show isn't quite as good as the Aladdin show that was there before but it is still cute. 
Sometimes on a hot day you just need to go somewhere indoors to eat (and have a cocktail). All of the restaurants in Downtown Disney have indoor seating that you can take advantage of but if you walk a little further you get to the Disneyland Hotel which houses one of my favorite spots to have a cocktail.  Trader Sam's is not only has an indoor space but despite the fact that it is a bar kids are welcome until 8 PM. The food is good there and the cocktails are even better but the atmosphere is the best.
 If you want to stay in the parks my favorite air conditioned place to eat in California Adventure is the Carthay Circle Lounge.  Sometimes there is a line to get in since they don't take reservations but if you don't go directly at lunch time the wait is usually no more than about 15 minutes.  The food and drinks here are fantastic. Get the tacos and you won't regret it.  An added bonus of a sit down place is that they bring you glass after glass of water to keep you hydrated.
If you want to stay in the parks my favorite air conditioned place to eat in California Adventure is the Carthay Circle Lounge.  Sometimes there is a line to get in since they don't take reservations but if you don't go directly at lunch time the wait is usually no more than about 15 minutes.  The food and drinks here are fantastic. Get the tacos and you won't regret it.  An added bonus of a sit down place is that they bring you glass after glass of water to keep you hydrated.  Another important thing to remember when the weather is hot (and even when it is not) is to wear sunscreen.  Even if you to stay in the shade and walk in the stores (another great way to get some air conditioning) you will still spend a considerable amount of time in the sun so protect your skin! Speaking of your skin, if you are like me and definitely don't have a thigh gap wearing shorts or a dress in the heat can present a unique problem. After walking all day in the heat you may notice your thighs rubbing together a bit too much for your liking and chafing isn't fun. I have found the best way to prevent this is with an anti-chafe stick like Megababe or BodyGlide.  You put it on like deodorant and it does the job. Like sunscreen you do have to reapply it in the middle of the day but if you are reapplying sunscreen anyway just add this in.
Another important thing to remember when the weather is hot (and even when it is not) is to wear sunscreen.  Even if you to stay in the shade and walk in the stores (another great way to get some air conditioning) you will still spend a considerable amount of time in the sun so protect your skin! Speaking of your skin, if you are like me and definitely don't have a thigh gap wearing shorts or a dress in the heat can present a unique problem. After walking all day in the heat you may notice your thighs rubbing together a bit too much for your liking and chafing isn't fun. I have found the best way to prevent this is with an anti-chafe stick like Megababe or BodyGlide.  You put it on like deodorant and it does the job. Like sunscreen you do have to reapply it in the middle of the day but if you are reapplying sunscreen anyway just add this in.
Final tip for hot weather in Disneyland: DRINK WATER!

 












