I will have more details on how to reach these places, as well as (perhaps) photos and more. You should be able to find these places using a decent map of the city.
Not for the faint of heart!
Here is a route I often take - it's 12km and nearly all pedestrian walkways & smooth. You'll need a map to follow these instructions closely, but it's very easy overall.
If you're in the gothic quarter, you can start from the cathedral. Go up Portal d'Angel (inland) towards Plaza Catalunya. Plaza Catalunya has a big round tiled area that is good to skate in at night (by day there are too many pigeons & people).
From Plaza Catalunya, head up Rambla Catalunya (it leaves from the uphill left side of P.C.) which has a pedestrian walkway in the middle of it after you get one block uphill from P.C. Get onto this pedestrian walkway and follow it uphill. After 10 blocks or so, you'll reach Av. Diagonal, which is a wide and busy street. You'll know you're there because the pedestrian walkway ends.
If you turn left onto Diagonal, you'll find a bike path you can blade on. Follow this for a couple of kilometres and you'll get to a big roundabout (Plaza Francesc Macia). Diagonal (and the bike path) continues on the other side of the roundabout. There are a couple of blocks that are not so smooth, but then it gets really smooth again for another couple of kilometres. You can go all the way to Palau Reial (where there is a metro stop on the left).
Then undo this. It's a great trip back - all downhill or flat. I do this route once a week or so and I like it because it's uphill out and downhill back and you don't have to worry about traffic (except occasionally & at the crossroads).
If you find all this on your map (it's pretty easy), the overall route looks like this
7 D \ i \ a \ g O o \ n \ a 6 l |\ Corcega <-----|-5 | |4 | Gran Via -----|-------> 3 /| | |2 | | | 1 | | 8| | | | | 9 ------------- 10 1 Cathedral 2 Portal d'Angel 3 Plaza Catalunya 4 Rambla Catalunya 5 A drinking fountain 6 Cnr. Rambla Catalunya & Diagonal O Pl. Francesc Macia 7 Palau Reial 8 "The Ramblas" 9 Columbus Monument 10 The sea