Stanbr and others have given very good advice ... I've experienced all these, but with limited use of a rental car:
• FERRY - That ferry from Heraklion is the least hassle, and I agree on the strategy of taking an afternoon bus there on Saturday - the wonderful museum's open late in July -- and enjoying a dinner near the causeway to the giant Fortress. A seafront room at a harborside hotel like Kronos is very handy.
• GORGE-WALK -- I did Imbros in mid-May and we almost had it to ourselves, maybe saw a dozen people; July of course would be different. Worth renting a car for the day especially in such hot weather, get car delivery the night before, to the free car park below Theotokopoulu street in Chania. .. then u can get early start, reach Imbros gate to begin your walk at 8:30 or 9 ... we didn't hustle, stopped for a picnic lunch midway, it still on ly took 4 hours, then a 1/2mile along a roadway to a taverna, where we paid some guy with a pickup €20 to take us back to our car. Imbros gorge is beautiful, I kept thinking of roast lamb, and realized it was the perfume of thyme bushes everywhere(!), near the end it narrows dramatically, with one spot where a long-armed basketball star might touch both sides. To me it resonated especially since i"d read how Cretans led British soldiers down this gorge to escape via sea, n Battle of Crete (and then sharpshooters picked off the Nazi pursuers). During the short walk at end, we were entertained by watching goats scale a completely vertical! cliff at the road's edge.
• BEACH -- Haven't been to Elafonissi, but have heard similar reactions to the midsummer crowding due to dozens of bus-tours. If you wanted a beach just to go to, and return to your Chania hotel, just to the West of Chania, there's a nice beach Chryssi Akti ... don't know how crowded it would be in July, due to hotels, but at least it's not as jammed with package-tourism hotels as some others. 2 friends & I had a wonderful experience at Falassarna beach in Late May, just a 50 minute drive from Chania. Two things to check in advance (1) this beach has an annual Festival sometime later in July (check later in spring for exact date) which attracts crowds; otherwise this very long beach never gets that crowded even july-August, according to TA reviews (2) check weather forecast, because a windy day can create large waves; also, for no waves, choose sheltered coves at N. end. One interesting option might be, cut Chania stay a bit short, rent that car & go stay 1 night in Falassarna .... Hotel Petalida at top of bluff overlooking beach has AMAZING sunset... and its owners also have fishingboat, serve their daily catch in their restaurant which is considered best seafood place in entire area. Then the next day u could spend driving North highway -- short stop in REhtymnon just for its FOrtezza then scram, also en route, the Memorable poignant Melidoni cave... then on to Bypass Heraklion to Knossos if u wish, and back to Heraklion for final Crete evening (see above).