You can ride to a great trail in <15 min from most places in town !
The "Foothills Trail" complex on the E side of the Res. will be just a few minutes away and is a great evening ride w/something for any level rider, gnarly to groomed, and no "big" climb. I'd try it your first couple of nights in town.
You can't go wrong at Horsetooth Mountain Park, if you are looking for something bigger, including the climbing. The climb to the Rock is more than the elevation of IL.
Weekends, the "Gulch Trails", Hewletts,Youngs and Dadds up the Poudre are super fun, no 'big" climbs, but a lot of water crossings, (maybe, we've been in a horrendous drought this year) and lot's of somewhat "level" techy sections thrown in.
If you really "want" to max out your suspension, head up to Crosier Mtn. just E of Estes Park. You can make it as "huge" as you want.
I grew up near Chicago but have lived here in Ft. Collins since '73. Altitude will be a factor, even up at Horsetooth your first couple of days and can even be lethal if you try to push it too fast higher up. I wouldn't try to ride any higher than 8,000' your first 3-5 day's here if you really want to enjoy your riding. As has been mentioned, it will be HOT ! 90-100, and dry. Devil's Backbone, Blue Sky, and the Foothills Trail have no shade at all. Figure at least a quart of water an hour on your rides. Another thing to consider, is that August is the monsoon season and many evenings may be a rainout. Higher rides are best done in the early AM as the risk of an electrical storm/downpour gets "likely" after about 1 PM. If you do decide to go high, it can snow any given day of the year, so be prepared for changable conditions.
Another thing, don't stay here too long or you'll be wanting to leave Chicago fast.
