There are lots of great clubs and dinning options right in River North--just west of Michigan Ave and north of the Chicago River. Sounds like you'll be staying just south of the river--near the Hard Rock Hotel.
Anyway, I recommend Quartino as a dinner option. It's a 'tapas' style restaurant--except with Italian food
http://www.quartinochicago.com/. Have only been there once--but the experience was great. Friends of mine are regulars there every weekend. Make reservations today--this goes for any trendy spot.
If you're looking for Mexican, De Cero Taqueria is really trendy. I have to warn you though... It's really loud in there. Too much so if you have quiet, mousy clientele
http://www.decerotaqueria.com/. If you're hanging with the loud bousterious crowd the food here is outstanding. De Cero is in the West Loop on Randolph. Seems like someone else mentioned this area. Very trendy spots on Randolph in West Loop.
If you're looking for a trendy Mexican restaurant in River North, I recommend Zocalo
http://zocalochicago.com/.
Phil Stefani's 437 Rush is my favorite place to get steak or seafood
http://www.stefanirestaurants.com/437rush.htm. Kind of a classic Chicago chop house decor and feel.
The club scene starts late and goes until the early morning. Most clubs that I'm familiar with don't seem to get crowded until 11 or midnight. Most are open late--some until like 5AM. I'm getting a little too old for this, but on occassion we're out all night. There's a great show at Vision Nightclub on Saturday
http://www.visionnightclub.com/...ails.php?article=404. This place is kind of confusing because you have Excalibur downstairs (the main entrance to the building) and Vision upstairs (entrance on the North side of the building). Within Vision there is Dome and Sky. Dome is typically hip hop and remixes. Not really cutting edge music typically. Sky has a bigger dance floor and usually the main act.
Sound-bar and Crobar are two other big venues (
http://www.sound-bar.com/ and
http://www.crobar.com/). Sound-bar is in River North and Crobar is near Goose Island--about a 10 or 15 minute cab ride. Have been to Spy Bar
http://www.spybarchicago.com/, too. It's just across the street from Sound-bar.
Anyway, these are just a few suggestions. There are so many great places to visit in Chicago. I'd just recommend staying away from chain restaurants that you can find outside of the city--except maybe Potbellies for lunch. It's like Subway--Chicago style.
Hope this helps. Enjoy your stay!