Risers: If you don't already have risers, installing them is a great idea. They allow for easy access to tanks during routine inspections and pumping.
Plants & Ground Covers: Things like bark, wood chips, and gravel are okay here. Wastewater is not treated in the soil next to tanks, so there isn't a need for plants. All the other advice still applies though. Don't put anything heavy over this area. If you do plant something, stick to shallow-rooted plants to avoid damage to tanks and pipes.