Since you don't have any specific breweries in mind, I'd get a day pass for the light rail ($5-9 depending on how far out of downtown you'll be) and stick to places within walking distance. Just off the top of my head:
Wynkoop - solid brewpub, big beer list, across the street from Union Station where you'll probably change trains, and which is worth a quick visit in its own right
Falling Rock - the (huge) original location is right on the 16th St pedestrian mall a few blocks away
Lowdown - maybe too hop-focused, but good apps/pizza and a nice patio, plus the owners/brewers are great
Declaration - huge beer list, food trucks, probably the best beer garden in town
If you do a little more walking on Broadway south of downtown around Declaration, you have Former Future (sours/barrel aged stuff), Grandma's House (average beer, hilarious decor), and TRVE (good beer, bring earplugs), any of which is worth a quick stop if only for the ambiance.
If you are staying on the west side (Lakewood, Golden) it's about a 20 minute drive to Tommyknocker in Idaho Springs.
Even on dry roads, that's more like 40-60 min. But the drive up US 6 is well worth it, and puts you right next to Cannonball Creek.
If you're coming up that far and have the time, might as well carry on to Summit County where you have seven breweries off the same exit.