There is an abundance of wildlife here. Everything from bear to skunks, foxes and deer. If you encounter them, leave them alone. If you are going to hike alone, it's a good idea to get to know their tracks.
Boulder is surrounded by open space and wildlife often makes its way into town. The closer you are to the foothills, the more careful you need to be about watching out for deer and other animals on the roads. This is especially true in the summer when the deer come to town to munch on all the watered lawns and plants after the hills have dried up.
In the fall, black bears can be seen on trails close to town. We often catch them eating out of our plum trees. Try not to disturb them. Especially if they have babies with them!
Also, mountain lions do live in the area. They are sometimes seen on the bikepath, and more rarely in town. Keep small dogs and children near you while hiking. They have a lot less fear of people than in the past.
Remember that you are at a very high altitude! When I first visited Boulder, I came straight from San Diego which is at sea level, and thought I was about to have a heart attack or something the first few days - I could'nt breathe! After a while, I realized that it was the altitude taking its toll.
If you are about to do something strenous like hiking, biking or rock climbing - be careful! Even if you are in great shape, it will take your body at least a couple of days to adapt.
Despite the murders of Jon Benet Ramsey and Susannah Chase a few years back, not much crime. Those are really the only instances of violent crime in the past 5 or ten years-that's why they get so much publicity (and the fact that they'll never be solved could have something to do with it too) Muggings are unheard of except for maybe on 'the hill' but I"ve never heard of one. It's a very safe place to raise kids and to be at all times of the day and night. Pickpocketing is non-existant but petty theft from vehicles, bikes and other little things happens. I would say now that your main danger in town would be to run out of money or have no place to park your car. Or maybe sunburn, dehydration, or just too much to do.
We've had a lot of mountain lion activity in Boulder this year. They follow the deer down into town and then attack pets that are left outside. So far this year, one cat and one pet goat have met their demise. The Division of Wildlife says the whole west side of town is mountain lion habitat. If you are hiking with small children, make sure that they are not in the front of the group or the last one. If you are on a trail that allows dogs, keep it on a leash. Actual sightings are pretty rare, but don't invite trouble.
We don't have many mosquitos, but the ones we have appear to be extremely good hosts for this viral infection. Make sure to wear insect repellent if you are going to be near water, or out in the evening or early morning. Mosquitos start in late summer and last until the first freeze (about September)
We have mountain lions in Boulder. Lots of them. Most of the time they are not a problem; your statistical odds of being nailed by one are very remote, nevertheless....
We just had a child who got jumped by a cougar here. The child will be fine but did need a trip to the hospital. If you see a cougar: Make yourself big , hands up high, use your jacket if you have one. If you talk to it, use a level tone of voice. If it is lying low to the ground ths is a sign it may be about to spring, in which case think about a weapon (rocks, sticks, whatever you have). If it does spring, fight back and scream as loudly as possible. If it shows no sign of springing or movement, try backing very slowly. Never take your eyes off the cat . (Bonus tip, if you are on skiis or snowshoes like a friend of mine was, you can't back up easily and shouldn't look so just stay put till the cat leaves.) Remember to tell our Dept. of Open Space where you saw the cougar and how it was acting; they like to keep track. All of this applies if you see a cougar in someone's back yard except that you call the cops.
When you are hiking here. you may well be under observation by a cougar. Stay with your friends and make sure kids are within 15 feet of you at all times where you can see them. Remember you have to see them to know if they are in trouble; don't count on them being able to yell (seriously, cats often hit hard from behind, there isn't any noise). Also, you need to reach them quickly to do any good. If you have dogs keep them within sight and voice command or better yet , on a leash. If you are hiking alone, it's better not to go at dawn or dusk. If you do go, banging on sticks or what have you is probably more of a way to draw attention to yourself. It may help with bears, who would prefer to avoid you, but has no proven value with cougars.
Finally: I'm not trying to make you paranoid, we've only had two fatalities in Colorado. Really, cars are far more dangerous....
Keep your dog on leash in Boulder in areas that are defined as "Leash Required." Some of these areas are the Boulder Creek Path, the lower part of Chataqua Park and certain hiking trails. Dogs are prohibited on the Pearl Street Mall and ignoring the signs can result in a huge fine.
the weather can change quickly here, so it's best to dress in layers and be prepared. You will often be warm in the city, and look at the mountains and see a storm happening.
Drink plenty of water when you are driving around Colorado. You can get altitiude sickness at any time, but especially during the first 48 hours or so of your adjusting. Water, water, water! :)