The project you propose is indeed do-able but the C64 in this instance would be more properly 
called a CONTROLLER.  The INTERFACE is what you have to install BETWEEN your C64 and the controlled 
equipment to prevent nuking your computer.  Your userport only sinks a few dozen milliamps 
(thousandths of an amp) at 5 Volt levels.  So it cannot directly control anything requiring higher 
current or voltage.
What you need is what are commonly called "solid state relays" that operate with 3-5 volt control 
voltage and provide solid-state relay action that controls various current and voltage combinations 
depending on what you want to control with it.  Various companies sell them, such as Digi-Key and 
Jameco Electronics, but they cost new more than your C64 is worth (about 20 bucks or so).  
Sometimes you can find them surplus for a few dollars each.
For VERY low current loads you can use Radio Shack or equivalent "logic reed relays" which utilize 
a coil winding around a small magnetic reed switch.  The coil is energized by the 5 volt TTL logic 
from your userport which closes the reed switch inside the coil.  These cost much less, about $2 or 
$3.00 but they handle only incredibly small loads, such as flashlight-grade lamps or very small 
radios etc.  They will weld themselves into the "on" mode or open up permanently like a fuse if 
loads drawing more than maybe .3 amp are operated from them.
Here's a typical safe way to interface the 64 to an external device:

                                               (Relay: 3-15 volt input, 120VAC output)
                                                 Crydom # CSD2410, example
_____  Userport pin C       + TTL input on SS relay______
     |---------------------------------------------|   ac|-----117 VAC hot input
C64  | Userport pin A (ground) -TTL relay input    |     |
     |---------------------------------------------|   ac|--------------(oo)-----117 VAC return
     |                                             |_____|          120 Volt Lamp,
     |                                                              Load, etc.
_____|

In this scenario, to turn on the lamp, type:
POKE 56579,255  (enter)
POKE 56577,1    (enter)
Now, to turn it off, type:
POKE 56577,0    (enter)

Other pokes for the userport:
POKE 56577,0 turns ALL 8 userport pins C through L OFF or logic low
POKE 56577,255 turns all 8 pins C through L ON or logic High
POKE 56577,1 activates pin C
"    56577,2 "" ""     ""  D
"      "  ,4  "        "   E
"      "  ,8  "        "   F
"      "  ,16 "        "   H
"      "  ,32 "        "   J
"      "  ,64 "        "   K
"      "  ,128 "       "   L

By The Way, controlling the "world" via your C64 userport in this way is ADDICTIVE and FUN!

Solid state (and other logic) relay sources: (try this web page!)
http://www.paranoia.com/~filipg/HTML/LINK/F_mail_order.html

No connection between any of these companies and me, of course, trying to ADVERTISE something over 
the internet would be SO icky.