Some people may consider it worth the trip just to see the largest wax begonia in the world. In any case, the American Horticultural Society is having one of those something-for-everybody festivals today. For gardeners, there will be lectures and demonstrations all afternoon on such specialties as roses, hydrangeas, ornamental grasses and bulb gardening. Horticultural experts will be around to answer questions. And there will be displays of medicinal and culinary herbs and - with luck - the last flowering of the Society's 5000 tulips. To start your own garden, they will sell lily of the valley pips, herb seedlings and bedding plants. And for children there will be free sunflower seeds to start them on their way to winning the grand prize in October's Super Sunflower Sunday. For meanderers there are the grassy acres of River Farm, which was once the property of George Washington. If the weather holds, there will be hot air balloon rides. And for the hungry, there will be an old-fashioned bull roast, the feast costing $5 for adults, $2.50 for children.

American Harticultural Society, River Farm, Mt. Vernon. Park on East Boulevard Drive. Today, noon to 5 p.m. Voluntary contribution for adults, $2.768-5700.