SYRUP: Combine sugar, water, rhubarb and strawberries in a medium saucepan. Boil over medium-high until the fruits turn mushy when pressed, 8 to 9 min. Strain, using the back of a spoon to press liquid out of fruit. Stir in lemon juice. Cool completely. Will keep, refrigerated, up to 1 week.
SANGRIA: To serve, pour the wine, syrup, club soda and sliced strawberries into a pitcher. Add mint leaves and ice. Pour into glasses.