The distance between Siauliai and Kryziu kalnas (Cross hill) is 12 km, so it is a good choise to use taxi. When you take a taxi from Siauliai, please, already agree with a driver that he will wait for you there and then will take you back. It is nothing bad if you agree from the beginning, that when waiting, he has taxometer off, otherwise a driver can use a situation and you will be charged for waiting. But as I told, it is not strange here to agree about price and conditions before you go.
But Siauliai tourism information center has good service with the taxies - 40 litas for driving both ways plus waiting for 30 minutes it's good deal. There is a shedule of busses going to Kryziu kalnas in their page. Only a thing, that you will still have to walk a bit from the buss station to the destination. In summer the center is working also on weekends - Saturday from 10:00 - 16:00 and Sundays 10:00 -15:00.
Siauliu turizmo informacijos centras
Vilniaus str. 213,
Tel.: 41 523110, 521105
e-mail: tic@ siauliai.lt, skype: turizmo_centras