Certifications can be double edged swords, especially in the computer field. They can be extremely useful to get your foot in the door, but you're probably not going to get much higher than 2x minimum wage to start. Figure around $30K average starting salary, most likely doing 1st level helpdesk or a GeekSquad type job if you want steady full time work. If you're open to contract work, you can make a lot more (in the $60-$80K range or higher) but the jobs are shorter duration and you'll have to go through the job hunt process every 3-6 months. Another downside to contract work is that your certifications need to stay current. If you land a job full time, you likely won't ever need to re-certify and you can build experience to advance.
As far as chances of finding work, a job search for "A+" certificates in the Denver area yielded about 10 potential jobs anywhere from $22K-$32K based on the ones that published salary info. The two contractor jobs were in the $40-$50K range, but they also had requirements for 3-5 years experience.
If you want better chances at finding a job, the medical field is always in high demand. A search for phlebotomy or nurse aides in the Denver area yielded 30+ jobs each. Pay is generally going to be a little lower and unlike the computer industry experience won't help build your career. If you want to advance in the medical field, you pretty much have to re-certify.
As for the others, things in the construction or CAD fields are pretty much worthless. Well, not worthless, but they won't help you get a job based on the certificate alone. They are really only valuable to people already working in that field to cross train or improve their skills. Especially with the current state of the housing market, you're not going to find a job in construction very easily. I can't speak much for the criminal justice or paralegal certificates. I only know one paralegal and I don't think she went the certificate route, she has a 4-year degree if I remember correctly. I can't think of anyone I know in the criminal justice fields except for one retired cop.
From personal experience, I got into the IT field through the certification route. I was already working in the field, doing 1st level helpdesk, which was really an entry level position. I BS'd my way through the interview and learned everything on the job. The 3 years experience I had in that job + an MCSE (back in 1999) got me a mid-level position in support (2nd/3rd tier), which I did for about 3 more years. That experience got me a more senior position, but still mid-level, at another company where I worked for 5 years building more experience and getting exposure to other areas of business. Now, I'm in a senior position where experience and accomplishments count more than anything else. I'm also back in school for a BS in CIS, and the classes are quite easy since it's stuff I've been doing for the better part of a decade.
From an employer's perspective, I've always been in the support arm of various companies. Desktop and network support for a company's internal users is a lot less demanding than support for the public. You'll have a lot more control and a more stable and standardized environment to learn in. For the people that I've been involved in hiring, certifications meant very little. They were never a make or break point for the job. A helpdesk manager I once interviewed had by his own admission very poor technical skills, but he had really good organization and communication abilities. At that time, that's what the job needed. Other people that I've interviewed, it's always been more about their rapport and thought process than technical ability. Actually, those who really stressed their certifications usually ended up not getting the offer. It has more to do with the attitude that they exhibited of "I learned it this way, so therefore it's the only way I will do the job" vs. "I learned this stuff to get certified and I can apply that knowledge, but I'm also willing and able to learn how you do it."
Still, as Ventanator said, you should still go for something you enjoy doing. A certificate or A.S. degree will get you more opportunities than you'll have without one, but still less than a 4 year degree or experience. Also, don't rely totally on a certificate to prepare you for a job. Every company is going to do things just a little bit different.
** Edit - I realized I didn't really address the way I looked at certifications on resumes and kind of jumped right into how I handled the interviews. When I looked at resumes for a tech position, a certification + 0 professional experience (most with no professional experience would often put school or hobby projects) was approximately equal to no certification + 1-2 years experience.
So, you sacked the cocky khaki Kicky Sack sock plucker?
The second cocky khaki Kicky Sack sock plucker I've sacked since the sixth sitting sheet slitter got sick.