Not just you, I promise. I am 29 weeks pregnant (not twins) and am at home with a 2yo and 6yo. I also have sciatica and PGP to contend with. They're currently playing in the garden (nice and cool at the moment) whilst I cook dinner (will be too hot later and I have energy now).
I have mapped out key things on the calendar for the holidays and DH will be taking 3-4 days off where we'll "do things". I'm also limiting out and about activities whilst very hot. Garden is set up with paddling pool, tent, table with umbrella, play house and scooters. Need to get sand for the sand pit. Freezer is stocked with ice lollies and ice creams. Probably have around 30 ice lollies in the freezer at the moment! Each day I open all windows and loft hatch (the latter cools the house down). I always have a hot water bottle in the freezer which helps me cool down! I also have a fan in the kitchen. Drink plenty of water. Do what you can to stay cool.
I also have a list of activities pinned up to help me come up with ideas (mostly sourced from MN over the years).....
Read books
Perform a play
Bowling with toilet rolls and socks
Dancing (youtube is great for this)
Play hair dressers
Treasure hunt
Board games
Indoor tea party
Make a tent/fort in the house
Movie afternoon
Arts and crafts (cutting/sticking, painting, drawing/colouring, make hm flowers out of flowers, handprint tracing/colouring in, paper mache, vegetable stamps, make a magazine) - do in the garden if you don't like mess
Play dough
Playing with boxes
Indoor picnic
Baking
Cress heads
Have friends over
Garden activities:
Water play
Cleaning pegs
Water balloon fight
Bubbles
Chalk drawing on paths
Paper on fences as an easel
Plant flowers
Make a miniature garden
Dens in garden
Picnic
Bike riding
Bird watching
Then of course there's trips to the park, perhaps with a picnic, trip to library, fly a kite. Then you have more adventurous things like soft play, cinema, swimming, pick your own, beach, bus trips, museums and bigger days out.
I hope that helps a bit. Try not to feel the guilt of not doing enough. You are growing people inside you!