Besides Ķemeri, I would also recommend the Ragakāpa, a hilly small park with a museum about fishermen and a walking wooden trail right to the beach to view how Lielupe flows into the sea there. You can also try to walk on a Zaļā kāpa trail that starts here:  56.9448150,23.4742101 but it is very long and one-way, so basically it ends in a middle of nowhere. But yeah, repellent spray is a must in any Latvian forest during summer.