could you go for a walk first thing in the morning when there are less people around, even if it is just round the block, just for the exercise, then try and do something at home which is interesting and sufficiently much of a treat that ds2 feels he has had fun and ds1 can do his own thing if he wants...build castle out of cushions, paddling pool or bath in the daytime with toys, baking, dance around the sitting room. Website with rainy day activities - I think Muminthemadhouse has lots of craft ideas.
The other thing is talking through any plan you have, well in advance, in simple terms, and make the plan very very simple too, like going to shop choosing a baking kit and coming home afterwards straightaway on foot. It is an ASD friendly strategy to talk things through so that new events are less alarming, in conjunction with making outings very simple at first, and possibly very familiar. With lots of positive reinforcement at destination, and short duration, bolt hole like insurance option (what will you do if he melts down or gets upset, think through how you going to manage that when out - McDonalds, home straight away, small reassuring item)
the frustration builds up that you have to do what other parents are doing with their kids over half term (museums and theme parks and visits), but sometimes it is easier just to do a bespoke plan that suits your family, a very simple limited plan.
I've been through this and come out the other side (ds now loves outings and in particular trains/tubes and football matches) and realise that my children were never ever going to enjoy museums (they still don't really, and as for parks...too much space and room for lying down on the ground and screaming or complaining or needing the loo etc) and realise I should have made things a lot simpler for myself and lowered expectations.
Dcs have seen most sights there are to see nowadays, London Eye, National Gallery, Saatchi Gallery, Berlin, Ireland, Lanzarote, camped, swum, pantomimes, theatre, restaurants. It will get better. No need to push them now.