Adding alternate options to an item in your quote is easy!
Simply Select the item you want to add options on, to open the sidebar
Scroll down in the item sidebar to click the link to add alternate items
Add other products and save.
When you preview the proposal, now you will a "View Options" link to expand the product so the client can choose the item they want.
Now, if you want to save for the future, don't forget to check the "Apply when adding item in the future".
Learn more about managing alternates on items