I was just there!!!! I went shopping on my way back from South Padre (actually past Laredo!) was shopping downtown San Antonio and checked out SeaWorld since Im going to be there next month for a birthday dolphin swim at Discovery Cove!
I would just take a rental car to be honest, the traffic is a nightmare and the roads confused me (I got lost!) but there are so many places to shop and a lot of cool buildings to see. My husband's dad is the assistant manager of the Texas Greyhound bus routing and he has always told me to avoid the buses because they stop in questionable areas and constantly make stops or constantly need repairs to where they are never on time. Ive told to never take the bus any time I go over to their place lol. I would just personally drive to Laredo anyways despite the boringness inbetween San Antonio and the coast. South Padre beach is close so thats something fun to do if you have spare time! It was very pretty there, and worth the extra drive
Also you might want to check out an airline I love to take! I love to fly with allegiant air just to get from point A to B, nothing fancy and ive had no problems with them, and they have a stop right in Laredo, TX. It might save you a driving trip if you can fly with them! Tickets are usually only $100 range round trip.
Anyways I hope you have a great time here in Texas!