spending a few days in Denver end of October. Would like to rent a (mountain) bike to explore and hit some trails. Is there moderately challenging trails within reasonable riding distance from near the convention center? If not, do I need to rent a car to get to them? any other recs for things to do? zoo, art museums, parks, botanical garden? thanks

I'm sure some with more knowledge of Denver can add to this but I can tell you that the Denver Museum of Art is great and worth a visit. There is a large REI store in town which I'm sure can help you with bike trails.

http://www.rei.com/stores/18

Denver does not sit in the mountains but on the plains. The foothills are about 20 miles to the west and another 10 miles to serious mountains. By the end of October most of the mountain trails will be snow covered except for right along the front range which might or might not be. The Denver metro area has about 500 miles of mostly paved and separate from roads path but not very challenging since Denver is pretty flat. You could ride from the convention center to Golden via these paths. The center of the trails is near REI at Confluence Park and you would have a lot of options from that point but use a hybrid or road bike as a Mt bike would be overkill. By the end of Oct we could have snow on the ground or hot and sunny so come prepared.

DS mountain bikes every week, and there is a reservoir maybe in the southwest part of Denver--maybe fmpden can supply the name. I think it is a bit hilly.
The Botanical Garden is a favorite of ours as is the Denver Art Mueum. Have lunch at Palettes in the Art Museum.

Just spoke to DS and it is Chatfield Reservoir. It might be a very long bike ride from downtown, but not impossible, I guess with the trails.

Actually two big reservoirs -- Cherry Creek to the south and slightly SE and Chatfield in the SW along with some other holding areas in the direction of Morrison and Littleton. The approaches to the reservoirs are along streams and therefore flat other than the slight rise as you go up steam. The biggest challenge is climbing the dam face which is a little steep but very short. Were you asking about single track? Probably need a definition of, "moderately challenging" as I would view all of the trails in the Denver metro area as very easy. You can link some long rides -- 50 to 75 miles. The Denver Bicycle Touring Club sells the premier trail map for the area. Available in all bike stores for $5. The Denver park department provides free maps but only covers trails within Denver city limits -- which is pretty small. And, again, you have little need for a Mountain style bike. I ride all the trails on a 32C slick tire.

Denver now has a bike sharing program with bikes stored at some 22 sites through out the downtown area and Cherry Creek. The first half hour is free and a buck for the next 30 minutes and then it gets very expensive. It is not a bike rental program but a point to point program. Great way to get around the downtown area. I think a one day pass is five dollars.

thanks for the help everyone. I would define moderately challenging (for me, anyway) as off road, could be single track, hills, not giant mountains.

You are going to have a hard time finding that in the Denver metro area. If you drove to the areas around Golder, Boulder you will find single track and some off road but you will need a car. Any of the bike rentals in downtown Denver can point you to the parking areas and provide maps. The vast majority of trails in the Denver area are off road but not single track. Also keep in mind that as a newbie in the area you could have breathing problems if you get to pushing the hills.