I lived in Boulder last year, and I can say the best advice is head to Golden or Lyons, either are 20 minutes drive from Boulder. Best trail on the front range (in my opinion) is Hall ranch in Lyons. Its best ridden as an out and back, and its one of the most technical climbs you ever will ride!
From Boulder, there is Betasso preserve loop, Walker ranch, and Marshall mesa. Non of them are fantastic, but they are great fun.
on a side note, depending when you're going, your best bet might be to head up to Nederland. the RTD bus leaves from Boulder and gives you 3000 feet of vertical to play with to get back down to Boulder. Magnolia road has some amazing downhills from it, and if you enquire at a local shop I'm sure they'll point them out!
Anyway, have fun – the climbing in Eldorado canyon is world class!