In Spain everything is a little more relaxed, so don't be surprised if you have dinner at 10 pm. Spaniards take time to socialize and create bonds. However, if you don't feel like staying up late, let them know. They'll definitely understand.
Don't expect your Spanish friends to arrive on time. Most likely, if you agree on meeting at 9 pm, they will arrive at 9:15 or 9:30 pm. Don't wait longer than an hour without a good excuse! That would be disrespectful and only a rude person would do it.
The most usual is for shops to be open until late in Spain. However, small businesses usually close around 2 pm for lunchtime and reopen at 4 or 5 pm. The best thing to do is to check the store's opening times online.
Pro tip: always kiss the cheek on your left first (the person's right cheek).
If you are both men the usual thing is that you do an informal handshake along with "¿Qué pasa, tío?" (What's up, dude?). In other cases, don't be surprised if some Spanish friend you just made gives you two kisses on each cheek to greet you.
The idea of the Spanish siesta is a little misconceived. Whilst it is present in the culture, few actually have time for it due to work schedules. It tends to be more common in southern regions because it's a good way to escape the midday heat.
Spanish streets do not sleep during the night, especially in summer. They are always full of people out and about. This is especially the case in city centres, residential areas and rural areas tend to be quieter.