Are you up for holidays that aren't in a holiday centre? If you have a car and don't mind driving down tiny rural lanes, I'd suggest Cornwall. If you stay in or near Looe, Polperro or Fowey, you could take them to Adrenalin Valley for really good Go Ape style adventures, then go to Monkey World to see the animals, go kayaking along the coast, go mackerel fishing, try a surf lesson, find a quiet cove and have a BBQ on the beach, and do the usual fish and chips on the beach, ice creams and pasties, long coastal hikes to burn them off.
Or a city break - bring them to London. Take a boat dfrom the London Eye/Westminster down river past Tower of London to Greenwich. At Greenwich, there's a choice of the Royal Observatory, the Maritime Museum, Queen's House, Cutty Sark, great park with pedaloes, vintage and craft markets, loads of nice cafes and restaurants. Then you can walk in the Victorian foot tunnel under the Thames, ride the DLR and get the cable car back across the river to catch the boat home. That's my favourite day out in London.
You could have a day in Kensington at Science, V&A and Natural History Museums (all free) have a picnic in Hyde Park or take tea in the cafe at Kensington Palace, visit Harrods.
Get tickets to a show. Wander along the South Bank to watch the buskers, have lunch at Borough Market, visit Tate Modern, mudlark on the beaches at low tide (not strictly allowed any more but people do it.) Or do one of the guided walks - ghost walks, Ripper walk, art, history, medicine etc - depending on their interests.
London can be expensive (especially if you include theatre tickets) but eating out can be very cheap and good, the museums and galleries are free and it's great just people watching.