Place mushrooms, zucchini and/or yellow summer squash, and sweet pepper in a plastic bag set in a deep bowl.
For marinade, in small mixing bowl stir together lemon juice, oil, sugar, salt, tarragon or oregano, pepper, and garlic. Mix well. Pour marinade over vegetables in bag. Seal bag. Marinate vegetables in the refrigerator for at least 8 hours or overnight, turning bag occasionally.
To serve, pour vegetables and marinade into serving dish. Serve with toothpicks. Makes about 8 (2/3-cup) appetizer servings.