We were in Katmai and Kodiak Island this year - as well as other parts of Alaska. You do not need a guide for Katmai NP if you only stay near Brooks Falls. The best (and busiest) time is mid July - that's when the Salmon are running. You are probably already too late to make reservations for the lodge at Brooks Falls. It sells out years in advance for the month of July. My advice would be to stay at their campground for a few nights. The best activity at Brooks Falls is late in the day and you have the best light then. The campground is fenced off with an electric fence - so no worries. You can eat at the lodge.
For Kodiak Island you need a guide with a plane to take you to the bear viewing areas. Kodiak was a very different experience from Katmai. It was just us (my wife and I) and the guide. No other tourists around. The downside is that you will not see as many bears compared to Katmai. Bear viewing at Kodiak Island is very expensive (mind you, Katmai is not cheap either!). For 4 hours we paid $1,000 for the two of us (incl. flight)- but it was well worth it.
I've attached two pictures to give you an idea what you can expect - the bear catching the salmon was at Brooks Falls, the other with the cub was taken at Kodiak Island.
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