It may be that you have the kind of autistic child that objects strongly to 'outside.' If you don't, just skip this and go and find something more relevant.
If you are truly unfortunate 'outside' also includes the garden. [translation = yard] If you find that attempting to take your child outside results in a serious case of the heebie jeebies, then you may also find that you and your child [ren] are trapped within the four walls of your home.
It is probably a good idea to try and find out what exactly is the true nature of their objection to 'outside.' This can be tricky if your child is also non-verbal. Some of it may be sensory in the realms of weather, temperature, the degree of light intensity and so on. This list is more or less endless, but again, difficult to pin down if language is not forthcoming. If you're happy for your house to remain your prison, all well and good, but even the more reclusive parent will find that on occasion, it is necessary to leave the house, if only for a few basic essentials such as food.
With that in mind, it is probably best to tackle the issue before it festers and becomes ingrained, the only other alternative being, that you will eventually leave your house in a six foot wooden coffin.
Now it may be that you are out numbered, one parent versus two children determined to remain troglodytes. You may be able to fool a friend into assisting you with this task, but failing that option, it may only be possible to deal with one child at a time. This is especially difficult, as it probably means that one child will be inside unsupervised, whilst you 'deal' with the other one outside. If this is the case place the inside child near a glass window or door with whatever the current obsession is. Whilst it is painful to admit that you are allowing one child to perseverate [push the ladder up on the fire engine, push the ladder down on the fire engine] for 20 minutes, this has to be balanced against the benefit of acclimatizing the other child to the 'outside.' Try and ignore the fact that the inside child is oblivious to the screaming agony of the outside child, as this is just a distracter to the parent. But I digress.
What can you do outside that might make being outside less agonizing or possibly more attractive? This depends entirely upon what you have to work with, as each child's unique make up will determine the outcome. For one of my children this meant lugging out Thomas the tank engine and his numerous cohorts into the garden and seleotaping them to the fence at sight level for a four and a half year old. Whilst I'd like to describe this as a treasure hunt with those pleasant connotations, the reality was more of a screaming rescue mission on his part. Clearly, this kind of 'game' requires setting up in advance and it's essential that the trains should be easily removable for those with poor fine motor skills. Ear plugs may be beneficial for the parent also.
For the other one, I found that the alphabet, shapes and numbers painted on the fence, paths, plant pots and other bits and bobs was a much better fit.
If you can make this a daily 'exercise' eventually you may be rewarded by the ability to have both children rescuing their respective preferences at the same time, therefore reducing the parental stress of leaving a child unsupervised in the house.
With luck, much, much later, they may begin to enjoy the experience. Perhaps, much, much later, it might become 'fun.'
I think most things have the potential to become 'fun' when they are no longer 'new.'