Hey there Iain,
Welcome to planDisney and thank you for your question! I'm thrilled to hear that you'll be able to stop by the Disneyland Resort in July. Even just one day at the Happiest Place on Earth will be filled with new magical memories!
While I always prefer having more than one day for a visit, you can certainly go on a "Greatest Hits" tour around the Disneyland Resort in a day - and I've done it before with my family when we tack on a visit to the end of another vacation.
My recommendation would be to make sure you have the
Park Hopper option, which is what will allow you to hop between
Disneyland Park and
Disney California Adventure Park during your visit. With this option, you'll pick a starting park with your
theme park reservation and then starting at 11:00 AM will be able to hop between both parks - subject to park capacities.
If you're also able to add the
Lightning Lane Multi Pass then all the better! With both of these extra options, you should be able to hit all of your must-see attractions. My recommendation would be to scout out a list of your top attractions before your visit and then start making your Lightning Lane return time selections as soon as you enter your first park of the day. After 11:00 AM then you'll be able to make Lightning Lane return time selections for both parks and hop between them as needed.
Additionally, if you really want to make sure you'll be able to experience everything you want to, then it might be worth considering purchasing the Lightning Lane Premier Pass, which grants one-time entry to each available Lightning Lane attraction in both parks for one day and you don't have to schedule the time. You can just show up, scan, and ride!
Both Lightning Lane options also come with unlimited digital downloads of your
Disney PhotoPass photos from your park day, which will be a great way to commemorate your trip. Make sure to take advantage of those by stopping by at least a few Disney PhotoPass photographers as you see them throughout your day.
Thanks again for your question, Iain! I hope you have a wonderful visit to the Happiest Place on Earth this summer.
Have a Magical Day,
Greg