A fool and his money are soon parted...
You have made 500k somehow so i'm sure you are not "average Kenyan" at everything..., i'll give you have a few options to process and mull over before you take a decision.
1. Stick to the business you know, reinvest in what is making you money now.
2. Open a fixed deposit or savings bank account (if you need easy access) and just sit back and wait for the right move / time, it will come to you eventually... Look around and simply open with the bank that gives you the highest interest rate.
3. Go to the money markets (Tbills etc.). Visit the offices of a reputable broker / investment banker, explain to him your plans and get some advice (make sure it's the free kind). Dont get into shares / long term investments with insurance companies.
4. If you have the guts and want to make money while you think, buy a plot in the larger Nairobi Metropolitan Area. If you (really) need cash after a few months, a bank MAY BE willing to lend you 200 - 500k depending on the land value.
"I'd rather be lucky than clever... every time!" - ME
"The problem is not what we don't know... it's what we know for sure that just ain't!" - MARK TWAIN
"Space we can recover... time never!" - NAPOLEON BONAPARTE