I've often pondered the crazy idea of zepplins lifted by hollow spheres with the air pumped out of them. Couldn't modern materials somehow be used to construct pretty light-weight spheres that could withstand the stress of containing a partial vacuum? Wouldn't a sphere with 90% or 95% of the air pumped out be lighter than one filled with hydrogen? Kevlar and aerogel with carbon-fibre struts.... yeah... Sure, mighty expensive (today - the prices of all those materials are dropping fast), but imagine what fun things one could do with a non-explosive zepplin with better lift.
Anyhow, checking the math of that is more than 5 min of surfing effort, so it remains a fun wild idea.