Hi Reshmi,
I agree with most of what Wonderful has already said, but have a few more suggestions.
1. Cordoba would be a great place to spend the night, but if you are short on time, the most interesting sights can be seen in half a day. High speed trains run at least hourly between Sevilla and Cordoba, so for greater flexability, buy your return ticket when you are ready to leave Cordoba.
2. Besides the Mezquita, be sure to check out the Jewish Quarter, Roman Bridge and Museum of Al-Andalus Life. If you are going to see the Alcazar in Sevilla, you can skip the Alcazar in Cordoba.
3. No need to buy hop-on, hop-off bus tickets in advance. Just buy them from the driver. I'd actually skip the Hop-on, hop-off bus in Cordoba. Outside of the Mezquita, Roman Bridge and Museum of Al-Andulus, Most of the interesting sights are within the old Jewish quarter, which the buses don't go into. You can easily walk from the Mezquita to the Bridge and Jewish quarter.
4. As far as I know, you can't book Mezquita tickets in advance. If you can, they are most likely being sold from a third-party site with a mark-up. Tickets are 8 euros, just buy them from the kiosk inside the Patio de los Naranjos. Mon-Sat, entry is free until 10:00 because of Mass.
5. Weekdays are typically less crowded then weekends.
6. high-speed trains depart from Sevilla's Santa Justa Station.
Jerez:
1. Yes, tickets should be booked ahead online - www.realescuela.org
2. Certainly. Trains run hourly between Sevilla and Jerez, taking about an hour. Just as with Cordoba, I'd take an early train and buy your return when you are ready to leave town.
