I want to quick vacation in a country, and then fly to another for a longer stay

As Bulgaria is in the EU, you won't need to self-quarantine but you will be subject to screening on-arrival.

From the UK GOV Spain Travel Advice:

Restrictions on travellers from outside the European Union and the Schengen free-travel area will be lifted from 1 July, providing the countries they depart from have agreements with Spain.

Testing/screening on arrival

[...]

• Provide the Spanish Ministry of Health with contact information and any history of exposure to COVID-19, 48 hours prior to travel. You can do this on the Spain Travel Health website or downloadable app. It is mandatory for all passengers travelling to Spain to fill out and sign the online form associated with your trip, regardless of your nationality or residency status. On completion, you will be issued a personal and non-transferable QR code which you must show (electronically or hardcopy) at airport health controls on arrival
• Temperature check
• Undergo a visual health assessment