I know you're not supposed to put requirements for dress on an insert, nor mention registries, but how does this sound? Is it bad? Just to give you a heads up, it'll be fairly casual.
Dressing up
While we encourage all manner of dressing up, please note that the reception venue is surrounded by grass (and there will be a bouncy castle) therefore high heels are not recommended, lest ye sink! As a couple, we will be dressing spiffily but comfortably, and will be encouraging you to dance the night away with us.
Gifts
We've been living together for several years now, and have toasters, cutlery and towels galore! Your attendance at our wedding is all we ask, and all we need for it to be a fabulous day. However, if you are feeling particularly generous, we would much prefer a donation into our charity wishing well, the total proceeds of which will be split between Diabetes UK and Mind, the mental health charity. Please do not feel obliged, however, as our day will be entirely complete with your presence.
Is this a rude way to mention it all? We're having a very family friendly, casual day. As for the gifts thing, we're doing pin badges as favours for Cancer Research UK with a donation in our guests names.