Okies, here's a couple ideas -
Leeds Castle - I understand it's really lovely there. Day trip with Southwest is about £22.
Leeds CastleHampton Court Palace - 2 for 1 offer on the price of getting in and a bit more local and yes, another palace/castle type thing.
Hampton Court PalaceSalisbury & Stonehenge - Even though I've done Stonehenge already, I'd be open to doing it again and I've not seen Salisbury yet. All in one tix available from South West Trains for £38.40
Salisbury & Stonehenge
Oxford - Caitlinn mentioned Oxford and I'd love to go. If 3 or 4 of us can get together to go from London Paddington we can get a group rate that will only cost £8.25 per person.
Group SaveCanterbury - Another place Caitlinn mentioned that I wouldn't mind going. A cheap day return ticket from London to Canterbury is about £17 and there are 2 for 1 vouchers available on admissions into some of the attractions about town.
Stuff to see and do in CanterburyI'm only listing things that we can afford right now. As much as I'd love to take a cheap weekend or day trip to somewhere in Ireland or Europe, it's not really feasible for us, so I'd have to take a pass on those adventures. But trips like the above would be ok and any others in this vein.