One of the first things to think about is the best location for the litter tray. It best to situate this in a corner as bunnies prefer to use their toilet with their backs to a wall but it also needs to be easily reached to make cleaning it out less of a chore. If you rabbit is new to its enclosure or learning litter training, then you may need a large tray till it gets use to it. Later on, you may be better off with a smaller corner litter tray as these will give your bunny more floor space. Corner trays can also be helpful as they have a high side which can stop some of the mess spreading away from the enclosure.
Once you have picked the best spot for the litter tray it best to put your bunnies food right next to it as this will encourage good litter training habits. Most importantly you should provide easy access to lots of hay, this is often best delivered in a hay feeder situated above their litter tray as bunnies will naturally poop and eat at the same time so this encourages good litter training habits. It's also a useful way of catching some of the loose hay that falls from the feeder to stop it spreading around. A food bowl should also be placed next to the litter tray that can be used to help contain food such as fruit or vegetables or pellet foods.
Your rabbit is also going to need access to water and there are two main option you can use a bowl or a water feeder. Bowls are widely regarded as the best as they are more natural for bunnies to use however it can be a matter of preference as some bunny moms prefer feeders to avoid poop and other things contaminating the water. Again, it's best to situate this near the food station and litter tray.
You are also going to need to provide your bunny with lots for stuff to do while you are away, and each toy is useful in their own way. Remember you bunny is also going to need room to move about so don't fill all the space and It's also best to keep a store of toys aside so you can rotate them to keep them interested when the novelty wears off. Each rabbit has its own favorite toys and behaviors so as you learn these you will know what gets ignored and what to get more of.
Chew toys are great for stopping bunnies getting bored. They need a constant supply of fiber to help their digestion and wear down their teeth that grow constantly thought their lives so having a few things they can munch on will help with this. Twiggy or grassy balls seem to be the favorite as well as grassy mats.
Toss toys are also a favorite toy for bunnies. In their natural habitat they would spend a lot of time arranging their warrens and customizing their surrounding so having a few things they can arrange, and chuck about will help satisfy this natural urge.
To help provide exerciser and a stimulating environment it can be good to provide a few platforms your bunny can jump on and off or a ramp they can run up and down. You can get bunny castles or ramps in stores or if you have some DIY skills it can be easy to make them yourself. Remember these will take up additional space in the pen so make sure your rabbit still has plenty of floor space to run around and stretch out.
Some rabbits like to have a bit of shelter that will make them feel more secure and give them a place to rest where they feel safe. There are lots of things you can use for this including pet tents, tubes or hideaways that will help satiety this need. You don't always need to spend money either as a cardboard box will make a great hideaway and provide hours of fun for your rabbit to chew and customize into a den.