I suppose you could try wall hangings, like they used to do before central heating. If you're getting direct sun through the windows, then curtains to block the sun would help. True insulation would involve getting into the walls.
In the summer, increasing air flow might help with humidity, and also temperature. Get a good fan.
Dehumidifiers do tend to use a decent amount of power, but after a point they're not luxuries, they are necessities. If you get into necessity territory, then don't skimp on it or you'll end up with much bigger expenses when you have to replace damaged items.