hire a car!! you definitely dont want to be bus-ing it on a sunday... you'd get to see a lot more sites under your own steam... I'd do avebury, silbury, WKL, waylands and the rollrights if I were you... a lot to fit in admittedly, but if you only have a day get up early, head up the M40 and back to HW down the M4 for a whistlestop tour...
t
xx