The persistent burden of parasitic infections and their established link to carcinogenesis highlight the urgent need for accessible, sustainable therapeutic alternatives. This study evaluated aqueous extracts from six agricultural byproducts—rice straw, wheat straw, maize stalks, sugar cane bagasse, soybean debris, and peanut seed coat—for their phytochemical composition, antioxidant activity, and biological effects against liver flukes (Opisthorchis viverrini, Opisthorchis felineus, and Clonorchis sinensis), gastrointestinal cancer cell lines, and inflammation models. Phytochemical screening revealed high levels of phenols, flavonoids, alkaloids, tannins, saponins, and steroids, with the peanut seed coat, rice straw, and wheat straw exhibiting the richest profiles. Antioxidant capacities ranged from approximately 45% in sugar cane bagasse to 77% in peanut seed coat. The same byproducts showing high antioxidant activity also demonstrated the strongest anti-parasitic effects, with IC50 values around 0.8–1 mg/mL against Clonorchis sinensis, Opisthorchis viverrini, and O. felineus. Anti-inflammatory activity, assessed by albumin denaturation inhibition, exceeded 90% in the most potent extracts and surpassed aspirin in efficacy. In anticancer assays, peanut seed coat, rice, and wheat straw extracts exhibited the strongest cytotoxicity across four gastrointestinal cancer cell lines, with IC50 values below 15 µg/mL. These findings highlight the potential of agricultural residues as rich, low-cost sources of bioactive compounds with significant antioxidant, anti-inflammatory, anti-parasitic, and anticancer properties.