Of course, I'd be happy to assist you with your travel plan to Spain.

Best Time to Visit:
The best time to visit these Spanish cities is from April to June and from September to November, when you can avoid the peak of summer heat and crowds.

Route and Accessibility:
You can start your journey in Madrid, then travel to Zaragoza, which is on the way to Barcelona. From Barcelona, you can conclude your trip in Valencia.
You can travel between these cities by high-speed train - Renfe, which is convenient and time-saving.

Altitude and Acclimatization:
The altitude in these cities is not high, so no need to worry about acclimatization.

Accommodation and Booking:
Try to book rooms in advance as Spain is a popular tourist destination. There's a wide range of accommodations to choose from according to your budget. Airbnb is also a good option.

Clothing and Essentials:
Given the mild Mediterranean climate, light clothing will do. However, it's always good to carry a jacket or a sweater for cooler nights. Don't forget your swimwear for the beach cities like Barcelona and Valencia.

Health and Safety:
Make sure you have travel insurance in case of any health issues. Maintain caution in crowded tourist areas to avoid pickpockets.

Local Etiquette and Respect:
Spaniards are generally warm and friendly. Be respectful of local customs and traditions. Spanish dining time tends to be later than what you might be used to, so plan accordingly.

Enjoy your trip and don't miss out on the local Spanish cuisines, Flamenco shows, and the beautiful architecture of Gaudi in Barcelona!