Pretty sure Zodiac office sells trips (also book swap)
you will see it on a little walk around,or just ask.
If not, then almost opposite, (well a bit down towards the corner) car and bike hire,money exchange etc,don't know a name.
But as woodruffe say's sometimes better diy ,a lot of trips are little better than the usual bus service !!
Can't say much more,because you don't really give any clue of the sort of thing you are interested in seeing,or places you want to go ??