You can actually add indoor functionality to your Garmin 305 by visiting a local center (if there's one local to you) and getting a series of metabolic tests...
I haven't done so yet, I believe the test cost $75.00 in my area. I'm interested in doing so, but have never made the time.
ETA: That link wasn't really helpful, you'd have to search around within the site to find what I'm talking about. Sorry! Anyhow, you look for a fitness center with New Leaf support - the search function for that is all the way to the right. The first link doesn't work for me with the search function, the second link's search function finds places for me. The center would give you a metabolic profile to upload into your Garmin.
Maybe this link is better: http://www8.garmin.com/products/trai...er/newleaf.jsp
Maybe this is the most confusing post I've ever accomplished!