I can't help with the tray, but Maclaren makes a Universal Bag that velcros onto the handles. This has a holder in it, though for water bottles and baby bottles. A resort mug would not fit. But I could put some toys or diaper stuff in it. It just couldn't get too heavy. But I took a little insulated bag for food (we took a 7 mo old) and a diaper or two (we were never out more than a few hours at a time) and stored those underneath. Then anything else we thought we'd need would go in the hanging bag and that included a water for us.
It's $20. I think it's available on their website. I stumbled on mine at FAO Schwartz in NYC where they have a Kate Spade/Maclaren section.
I enjoy it for small day trips.