While on vacation in Steamboat Springs CO I fell in love with a road or trail bike I rented and so did my family. We road for miles on a trail and it was great. I want to convert the mountain type bikes that I have in my garage that are falling apart. They need new brakes, new seats, new tires, and possibly gears. Is is worth it or scrap and start over?
You can put smooth tires ("slicks") on a mountain bike wheel rim instead of the usual "knobbies". These slicks can be in the typical 1.95" mountain bike tire diameter, or you can go narrower down to 1.75".
Regarding the bikes in the garage which needs brakes, seats, tires and possibly gears; those are not very expensive items. Tires are about twenty dollars; used seats in good condition can be had for a few dollars.
If you'd like higher gear ratios, it is not expensive to swap a casette or freehub. Find out which one your bikes use. Probably, I would guess, these mountain bikes have thread-on freehubs that remove with a FR-1 tool (which should cost under $10, and needs a big wrench to go with it). A new freewheel hub costs about $20 to $30, and gives you an opportunity to increase the gear ratios. The swap is a five minute job.
However, once the repair budget for some cheap old mountain bike surpasses about $50, it quickly stops making sense. Instead of spending $100 on maintenance, you can get something newer and in better condition for $75 and spend $25 on it, ending up with a better bike.
How much is your time worth? Many DIY-ers make the mistake of treating their labor as being free, and only counting the cost of tools and materials! "Look at the bike I put together with a $100 budget!" (And with only 20 hours of my "free" time put in).
I suspect that it is not likely to be cost efficient, as the end result would not match the experience of whatever bikes you rode on vacation.
In particular, you should be aware that, even if you change every component on a mountain bike, the frame will still have mountain bike geometry. Geometry has a large effect on your riding position, bike handling and the overall "feel", which seems to have been a factor in your enjoying the bikes on vacation. For example, putting slick tires on a mountain bike will reduce rolling resistance slightly, but you'll still be in the same riding position and the bike's handling won't feel drastically different.
Huffy bikes are generally quite low end bikes as well. You didn't specify the age of the bikes, but it sounds like they've been sitting around unused for a while. If you take off the components you mentioned, then essentially what you will have are a low-end out-of-date frame and wheels. If you can get the bikes back into working order without investing too much money, then I'd say just ride them and fix them until they die.
A final consideration is whether the bikes are actually the right size for you as well: a correctly sized bike which has been adjusted for your individual dimensions makes a world of difference.