Help How do I get from Houston to Galveston???

We just booked a cruise leaving out of Galveston to the Carribean. Since we booked within 10 days of departure we are no longer able to book transportation through Royal Carribean so we are told. We've called and can't locate a car rental agency that will allow drop off's in Galveston from Houston. As far as we can tell from the website Super Shuttle doesn't operate from here. Short of spending loads on a taxi does anyone else have any ideas? (We also face the same deliema on the return Galveston to Houston) Right now we are scheduled to arrive the morning of cruise departure around 10am and depart from Galveston at 6pm (with check-in at 3pm) We are somewhat flexible on flight time and arrival city. HELP Please? Thanks. We also plan to spend time in Galveston on the way back ~ what are the must sees?