Next to Riga Main Railway Station get in tram nr 11 direction to Mezaparks, get out at last stop Mezaparks (you can blackride). Find main entrance of Riga zoo and at the left side from entrance there is forest park with big lake where you can swim. Go inside in bushes in distance from roads becouse this is popular place with lot of people. There is police cheking main park roads for safety so camp in distance from roads.
Another option might be Zaķusala, the island where
LTV and its tower is located. It's within walkable distance from city center and really quite place with nothing else really there. Both tips of island are covered with bushes and trees so it is easy to find a place to hide away.