To be honest each away trip has its own advantages and disadvantages, in a vague, completely subjective order these would be my favourite trips.
1) Coventry - we always take a reasonable number there and the games tend to be classics. Despite the reputation it is generally a friendly place to visit (leaving aside a bit of excitement last year...) and our record at the Sponsordome is pretty good. It is also the closest away trip to Cardiff and for me it combines a good atmosphere with a decent pub and an exciting game. We are allocated the block behind the goal and generally fill that block (and occasionally another one as well)
2) Hull - strange choice perhaps for second place but Hull's "Arena" is an old style barn which allows you to be as close as you want to be to the players (indeed they traipse past you to get to the changing rooms) Despite smaller crowds than some, the atmosphere is pretty good and we generally win there. The down side is that the number of away fans can normally be counted on your fingers...
3) Nottingham - great arena, great location, lots of away fans but, only my third favourite away trip due to our overall record there and the fact that it is a little too "corporate" for me. The best facilities by far but the away block is tucked away in a corner so you sometimes feel a little bit disengaged from the action. Sitting in the front row is recommended though. I was there when Voth checked Coach Corey and I honestly thought that they would end up in my seat.
4) Sheffield - great fun if we win, dreadful if we don't.... Strangely one of the friendliest places to go to but as it is rarely half full the atmosphere is often dire, you have to listen to Simms for 3 hours and it is a long way. Still great if we win though.
5) Scottish trips - great fun, especially during double/triple headers
Haven't been to Belfast so can't comment on that trip.
Not sure how the ticket situation works with Bob but, I would recommend booking in advance for Coventry, all other places are fine on the day although Nottingham's queue's are legendary...