We have slowly acquired a small self storage facility, an 8 unit apartment, completed a large rehab project, and turned our previous personal house into a rental after moving into the rehab project. 

@Michael Palma  Mobile home parks are second ONLY to storage facilities with ease of financing.   

Is there opportunity for additional bedrooms, adding storage, things like that?

Several options ..Could you build a storage unit on the lot or could you sell the lot to someone who would like to build a house on it .

Larger box spaces tend to take longer to lease out as bigger companies are slower to move and rent is typically higher compared to a larger pool of smaller tenants going for the regular type spaces.National tenants tend to want brand new buildings with higher ceilings for storage and shipping.

We have also done as much creative storage as possible.  

It's an unfinished basement that is used for storage, laundry, and HVAC.

Or are there other ways (parking, laundry, storage) etc... but key is to leave some meat on the bone for future investors.

If you are an Accredited Investor, you could place that 100K with an experienced sponsor in a real estate syndication in any number of asset classes (Large Multifamily, Self Storage, Mobile Home Parks, Assisted Living Facilities).

The goal is to pile up enough cash to buy large value-add multi-family, storage units, mobile home parks, not exactly sure yet.