I did the bike tour in Aruba.... and it can be fun... but it can be a physically draining tour... it is about a 10 mile ride from the natural beach to arashi beach/california lighthouse. Most of tour is on the same paths the dune buggies and 4x4 jeeps follow... and most of it was sand at the time... so it was a challenging ride. I know I needed to take a shower after I got done! Can be fun.. but you are on a tropical "desert" island... so it can be very hot and there is no shade.
Didn't do any tours in Willenstad... actually just did alot of walking through the market area and the floating draw bridge. Be sure to hit this area and walk back to the floating market area... very neat and something different you will not see here in the states. There is also the swim with dolphin tour in Curacao