I heard that the new features don't appear on old saves. Is this true? If so, is there a way to turn it on? I'd like to keep my old world and just travel to a new area for the new stuff.
If you travel far enough so that new terrain has to be generated then you should get some of the new features (swamps, vines, ravines etc.) in the newly created chunks.
I wouldn't like to say whether strongholds will be generated or not as their location appears to be related to distance from your spawn point.
By default, worlds created prior to 1.8 will be Survival maps without structures enabled. If you use an NBT editor (like, er, NBTedit), you can enable NPC villages, etc., by changing the value of Map Features from 0 to 1. (You can also toggle between Creative and Survival by switching Map Type between 1 and 0.)
However, even with structures enabled, new map features and new biomes will only appear in chunks that haven't already been generated, which means they only start generating beyond the furthest points that you've traveled. Since I'd explored a much larger space than I'd built in, I solved this problem by opening my world in MCEdit, using the selection tool to draw a big box around everything I'd built, switched to the Chunk Tool, and selected Prune (which deletes all the chunks outside of the ones you've selected). When I opened the map again once I'd updated to 1.8, the new map features and biomes (including, I can guarantee, mines) appeared in the areas outside my selection.
With this technique, the boundaries can be a little bit jarring, so you can use a script like this one to improve things, although I haven't tried that myself.
Features that are in Minecraft 1.8 will only appear in worlds created in a older version if you travel to a new chunk.
Will new Minecraft features appear in existing worlds?'s answers have some more information about it.