crags for learning aid

Anyone know where the best crags around Denver/Boulder are to learn basic aid techniques? I'm looking to set up solo top rope systems and will need to be able to access the top of the cliff. Looking for a good mix of cracks to place gear and some bolts to practice lowering out, etc. without jamming up other parties. Thanks!

I don't know if it's the best, but my partner and I worked out the aid climbing process on Country Club Crack, on Castle Rock in Boulder Canyon. We went in the winter so we weren't impeding any other climbers. It's steep enough, the crack is continuous (after the first pitch crux) and you can walk off.

Happy Hour Crag in Boulder Canyon has easy top rope access and a few good (but short) cracks to practice C1 on. Also second the Country Club Crack recommendation if you go on a day when there won't be anyone else. While you're at Castle Rock, practice aid roof is a good chance to flail on a mega steep bolt ladder 

Castle rock is the best place to learn aid in boulder. With CCC and the lines on the aid roof. China doll in the adjacent dream canyon is the only real aid route around. Pitch two the one that goes free has bolts so it's a good place to practice C2+ moves with out any danger. The rest of the route hasn't gone free and is mind blowing super steep relatively easy aid. if you want to feel what hard aid is like, canary by pass in the red garden roof routes of eldo is an amazing C3 and harder than most A3 C3I've done on el cap. kors bolt ladder through the roof is wild! and the first move, a reachy nut equalized with a micro cam right above the ruper Ramp is full on! Other than that just drive out to the desert and play around on some obscure aid stuff in the San Rafael swell (have a hammer and some pins for the swell) or do a small Zion wall like space shot, prodigal or moonlight.  

Another set of options that see less traffic than CCC are Roadrunner and .30-6 at Wall of the 90s in CC (Golden).  Might be a tiny bit of mandatory free in them, near the beginning?  Good aid cracks for sure.

I believe that's how Practice Rock in Boulder Canyon got it's name.  The finger crack is easy to top rope and makes a nice intro to aid climbing.  As a bonus, when you're done practicing aid, it's a super fun trad lead.

Go to literally any sport crag.  Clear Creek and Boulder Canyon are perfect for this.  So what if you are aiding on bolts.  Most people get bogged down aiding because their systems aren't efficient, not because they can figure out how to place a micro-nut.  You can always add the gear later.  Sport areas give you every kind of terrain you would find aiding.  Roofs, traverses, slabs, etc...  Practice hook moves between bolts.  Traverse over a route or two if the crag isn't crowded.  Pendulum over to another route.  Get creative.  A lot of places also force you to do a free move or two between bolts.  This is also great practice for getting out of your aiders and moving quickly.  The other advantage with bolts is that you can just get more mileage in than messing around with gear while you are getting your systems dialed.  This has been the best help for me personally to get ready for aid objectives.  Just be ready for some odd looks from the sport climbers.

Plus 1 to what ROC said! Do a few bolts ladders to get the flow. Then go somewhere (preferably not Country Club) and plug some cams. Practice rock is good and there's another route on I think Cob rock to do some hooking.