Well your favorite trout streams are running hot this time of year and fishing these streams will jeopardize the livelyhood of the trout that you love to cacth. So what is a die hard fly angler to do? You pretty much have two choices if you want to wet a line. Head to the spring fed mountain streams, or step out of your comfort zone and cast a line on a lake, or river.
Warm water angling is a blast and you really never know what you may end up hooking up with. Personally I think the best warm water flies are small streamers. Size 10-6 simple bugger type streamers will catch a larger variety of fish species. Your presentation does not have to be as perfect with warm water fish as it does with trout for most of these species which is also a bonus. Bass, bluegill, perch, and crappie are fairly easy to catch and you can really rack up some numbers with them. Depending on your target species 50-100 fish days are not uncommon.
Other fly options to consider are your standard trout nymphs and dry flies. The bugs are still in the lakes and rivers as well and pan fish especially will take a large variety of these insects. I have had a lot of success using my lively legz nymphs and soft hackle flies for panfish. I fish them almost like how you would nymph fish. You can also use indicators and personally I think the extra splash draws attention to your bugs. Feeling a nice pumpkinseed sunfish on the end of your 3 wt is awesome. That first burst of adrenaline the fish has when it realizes it is hooked is a rush.
Another species that I feel is just fantastic on the fly is the perch. In the past I have only really fished for perch with bait with a boober, so feeling how hard and intense they hit flies was quite pleasing. They are like mini walleye but without the teeth. The initial tug is like that of a wild brown crushing a streamer. When a perch hits your fly they pretty much engolf the whole thing so be sure to have hemostats with you.
I would say your easiest larger fish to catch on the fly would be a largemouth bass. Bass will take streamers both large and small, poppers, nymphs, and all sorts of flies. Ive even caught them on mop flies. My favorite way to fish for bass is with my 10' 3 wt rod and small streamers. Now if you want to chase the big boys I would suggest a minimum of a 6 wt, and ideally an 8 wt fly rod. Mainly size your rod, and line to the size and weight of the flies you will be throwing. Bass are a total blast on the fly. Most of the smaller bass I target will still peel drag on the 3 weight, and put a nice bend in the rod. They will become acrobats with leaps clean out of the water to get the fly out of their mouth. It is quite the rush.
To me the most exciting thing about warm water fly fishing is that you never know for sure what you may hook into. When I'm targeting trout I can pretty much be sure it will either be a trout, a sucker, or some member of the minnow family that takes my fly. While warm water fly fishing, that unknown is what gets me. I have had times were I was using a peeping caddis nymph for bluegills and hooked in to a 20+" walleye. This was an epic fish to have on the end of 3wt nymphing rod, let me tell you. I didn't get to land him as I had to hoist him up over a large wall of a spillway, but seeing my peeping caddis nymph in his mouth blew me away. I've also hooked into to large carp, and have even caught a bullhead catfish on fly gear.
So if you find yourself pouting in the summer because you can't fish for trout, you are really missing out of a lot of relaxing and fun filled opportunites to land a wide array of fish on the fly. Suck up your pride, and give warm water angling a try, you just might fall in love with it.