There isn't a lot on in the Union on a Tuesday night so it's probably not the evening to see it at it's best. You could always start your night in the Union though, somewhere like the Terrace Bar or the Old Bar. Both are good.
When you get in to town there's always LOADS on a Tuesday. If you want to see one of the bigger student nights, Tuesday @ Space is definitely a good one (see above), Tiger Tiger again (but it's probs not the best) or just head to a bar like Varsity, Dry Dock or Risa.
Have a good night as I'm sure you will!