Use pt 2x4's or 2x6's across for your decking over a frame is easy. This is pretty permanent though unless you have a place to store it. We have done this to get pianos, bass amps, etc. in and out of our basement on a much larger/permanent scale. On the level runs, we used 4X4 rails, build a slanted frame out of 2x4's for a ramp. You could build just the ramp part to get over the curb. It's held up amazingly well and conveniently removable if needed. We've had it 12 years now, only replaced 2 boards so far.
Complete directions: http://www.matadorcoupe.com/seebee65/Boardwalk.htm