1. Purée raspberries in a food processor. Strain through a fine-mesh sieve, discarding seeds (should yield about 2 cups purée). In a large pitcher, mix the purée, sugar, lemon juice, and salt (you can make the purée up to 12 hours before serving; just keep refrigerated). Add the seltzer and stir until combined.
2. Place a few small scoops of ice cream and some blueberries and other berries in each of 6 glasses. Fill with the soda. Serve with long spoons and straws.