A picnic is best done in autumn, winter or spring, when it rains less and you don’t have to worry about the weather being too hot. Ideally, you should look for an area that offers semi-shade under big trees, with some sunny spots for the winter. The best time for a picnic is lunchtime (if you have an afternoon ceremony, you end up having your picnic in the dark), which means your ceremony should start around 10 or 11am. A garden ceremony would complement the picnic reception very well, but an outdoor chapel is also an option. You should consider providing cushions or some garden style seating for elderly guests.
Since there is no dancing, you don’t need a DJ. You may consider running background music, but this is really optional and not a necessity. Typically your reception will be shorter because there is no “evening entertainment” program. It could be a great idea to leave for your honeymoon that same afternoon or evening.