The drive to the west coast is spectacular. Wanaka is a very likeable, calmer version of Queenstown. There's not all that much to do around the town but perhaps wine tasting (the nearby hikes are unshaded and heavily tracked by sheep/horse dung). But the lake and beach make for a great leisurely day. A swim in the midday sun is good fun and far less cold than the glacial-melt streams I've been jumping in recently.
Campervan'ing is allright, though holiday parks seem to cost about $20 per night and backpackers cost about $25, and that means a bed and shower/kitchen (shared). You can stay for free at DOC campsites that are about 15k outside Fox and Franz Josef townships, but they have a suggested $6/night. All prices per person. The campervan isn't particularly comfortable for the longer legs, either. A car+hostels seems to be the better option.
The activities consist of hiking, eating/cafes, or helicopter flights, which means the options are hiking or eating/cafes unless you're willing to pay rates that are probably more expensive than a fancy prostitute for the helicopters. And the hikes are either short or very long, spending most of your time in the jungle.
The jungle is fascinating and teeming with plant life, but when a 7 hour roundtrip hike with great views of the top is spent 90% in lush trees/brush while climbing steeply, the view payoff is a little strained: You're sweaty and hot and have been looking at the same trees for 3 hours. And then you get to go straight back down them. This is the Alex Knob hike at Franz Josef. Climbs 1.3km vertical for the views.
Most of the west coast is wilderness that's nigh-inaccessible due to the density of the jungle. It's beautiful, but quite limited except by air travel. Of course, your experience may differ.
There are heli/plane tours that fly you by the Fox Glacier, Mt Cook, Tasman Glacier, Mt Tasman, and Franz Josef Glacier all on the same trip. They really are that close together! All within about 30k, even though driving from one side to the other is a 450k drive. I made a map to show, click here to see it.