The best way to get around Lisbon is by public transportation. The metro system is extensive and covers most areas of the city. A single ticket costs €1.50 and can be used for 1 hour of travel. A 24-hour ticket costs €6.40 and a 7-day ticket costs €12.15.
For the day trips to Sintra and Cascais, trains depart from Lisbon's Rossio Station and run regularly throughout the day. A roundtrip ticket to Sintra costs €4.50 and takes approximately 40 minutes each way. A roundtrip ticket to Cascais costs €4.50 and takes approximately 30 minutes each way.
If you have more time, consider adding a day trip to the medieval town of Óbidos or the surfing town of Ericeira. If you have less time, consider skipping the day trip to Cascais or Sintra.