Combine rice vinegar and sugar in a medium bowl; mix well. Score cucumber lengthwise with a fork. Cut in half lengthwise; scrape out and discard seeds. Cut into thin slices; place in bowl with vinegar mixture. Add onion and jalapeno; toss lightly. Cover; refrigerate at least 30 minutes or up to 8 hours before serving. Season to taste with salt.
Total preparation time: 10 minutes (does not include marinating time)
Per serving: 46 calories; 0.5 g protein; 11 g carbohydrate; 0.10 g fat; 0 mg cholesterol; 243 mg sodium