I'd hazard a guess at most sites and topics that are not directly related to summer activities. So if it's not sports, leisure, sunburn advice, holidays, daytrips and so forth, it could be at risk of the slump.
Add some of the above topics.
Yes. Less people will be online when the sun comes out. You can't fight nature.
I just keep building and writing content. I could build another summer-seasonal site, but that's not really my thing. Although income will always ebb and flow to an extent, the summer slump tends to balance the Christmas rush. There's always work to do during quiet periods to get ready for this.